The gnss_crs_spec is EPSG:9755, i.e. WGS84 (G2139).
The name for the spec string "+proj=latlong +datum=WGS84" is changed
from geographic_crs_spec to ballpark_geographic_crs_spec, because
it is retained for compatibility.

Replace Georeferencing::ballpark_geographic_crs_spec with
Georeferencing::gns_crs_spec in all occurrences that relate to Mapper
internals. The former refers to the long-term evolution of WGS84,
while the latter to an up-to-date realization G1762.

Georeferencing::ballpark_geographic_crs_spec continues to be used
for the map file format, for interoperability with older versions
of Mapper.

In OgrFileImport::setSRS that sets the transformation of imported
coodinates to the map SRS, if the source SRS is being set to WGS84,
substitute Georeferencing::gnss_crs_spec. This makes the resulting
transformation more accurate, for example if the map's datum is NAD83.

With Mapper now using the WGS84 (G2139) realization for templates and
as a pivot for coordinate transformations, template alignment has
changed, causing differences in the test results. The test cases
    TemplateTrack NAD83
    OgrTemplate NAD83
in ogrTemplateTest no longer fail, because the tracks are now aligned
well enough with the map and its datum to pass the test. In addition,
TemplateTrack and OgrTemplate now align tracks the same, causing those
same test cases to pass in the templateTypesConsistentTest.

On the other hand, the test cases
    TemplateTrack from v0.8.4
    OGRTemplate from v0.9.3
in ogrTemplateTest now fail, because behavior of earlier versions of
Mapper did not handle track coordinates as accurately. The position
is changed by 0.3mm on the map, or 1.3m on the ground.

This changes Georeferencing::gnss_crs_spec from EPSG:9755 to
EPSG:9057. PROJ handles EPSG:9057 with with an epoch several years
earlier than EPSG:9755, which makes coordinate transformations to
NAD83 less accurate, by 20 cm.

With EPSG:9755, Mapper was failing tests in Azure pipelines.
Changing to the earlier EPSG:9057, which indicates the
WGS 84 (G1762) realization, may work with the older version
of PROJ that's available in the superbuild.

TemplateTrack will always rely on the map's Georeferencing to convert
GPS tracks to map coordinates, which can now be a more accurate
realization of the ballpark geographic CRS. At the same time, to save
the realization CRS in the file, or to keep it in the TemplateTrack
object would provide no benefit.

Each Georeferencing object is given its own geographic_crs value,
so that Mapper will be able to support older maps without affecting the
alignment of their templates, while simultaneously providing more
accurate positioning of georeferenced templates in new maps.

For this purpose, ProjTransform objects are enhanced to have an
explicit geographic CRS in addition to the projected CRS.

This modification to ProjTransform is unsuited to another scenario to
which ProjTransform used to be applied, namely when the ProjTransform
would do no more than indicate a CRS. The mechanism supporting
'forward' and 'inverse' methods of such objects was not constructed. A
new struct, 'ProjCRS' is provided for this specialized need. Splitting
off ProjCRS from ProjTransform provides clarity to see which of these
utility objects define a transformation from a geographic CRS, and
which define just a CRS.

The static value Georeferencing::gnss_crs_spec is made private to the
Georeferencing class, so that non-core uses of the map's geographic
WGS84 CRS will be sure to depend on the map to obtain the CRS, rather
than specify a particular variant of WGS84. Their references to
gnss_crs_spec are replaced with calls to getGeographicCRSSpec(), which
in the future will depend on the map. The Georeferencing's
toGeographicCoords, toProjectedCoords, and toMapCoords methods will be
consistent with the CRS returned by getGeographicCRSSpec().

The isDatumBallpark() method indicates compatibility for working with
old maps. It enables GDAL import to provide compatibility with the way
old Mappers aligned template tracks. When compatibility is not called
for, track import is tweaked to use a more accurate geographic CRS.

The attribute 'is_realization' is added to the Georeferencing class
and to the geographic_crs_spec of the xml file format. A 'true' value
indicates the new, more accurate geographic CRS, while 'false'
provides the same geographic CRS as earlier versions of Mapper.  The
geographic CRS is used as pivot when transforming from one version of
map coordinates to another. It is also used for GNSS tracks and for
display of geographic coordinates.

The Georeferencing::setDatumBallpark() method provides for the
compatibility mode to be turned on or off.

On loading a map, the user is warned if they haven't decided whether
compatibility is wanted. When the user opens the georeferencing dialog,
a message box asks the user whether they want extra accuracy, or
alignment that is compatible with older Mappers.

Example and test maps are updated to indicate the 'is_realization'
attribute. The template_t test had two test files as examples of what
older versions of Mapper would save. This commit provides the
compatibility which makes the corresponding two tests succeed.

Adds a test map that has a geographic CRS with is_realization="false".
This is used with TemplateTest::ogrTemplateTest to check support for
template alignment that's compatible with Mapper v0.95.

Also, for FileFormatTest::saveAndLoad, getGeographicCRSSpec() is
added to the map comparison.

Instead of saving _that_ the map was developed with a recent
realization of WGS84 for its geographic CRS, save the _spec_ of the
geographic CRS itself.