Add methods to check geometry type and convert simultaneously

[peterstace]

Description

• Renames the AsXYZ methods of the Geometry type to MustAsXYZ. This
  follows the go convention that methods and functions prefixed with Must may
  panic if preconditions are not met. Note that there's no change in behaviour
  here, it's simply a rename (these methods previously panicked).

• Adds new methods named AsXYZ to the Geometry type. These methods have the
  signature AsXYZ() (XYZ, bool). The boolean return value indicates if the
  conversion was successful or not. These methods are useful because they allow
  concrete geometries to be extracted from a Geometry value, with the check
  for the type and the extraction only specified once. Previously, users would
  have to first call IsXYZ to check if the type is expected, and then
  conditionally call AsXYZ. Users now just have to call AsXYZ, and can
  check the flag. This helps to eliminate the class of bugs there the type
  specified with IsXYZ differs from the type specified by AsXYZ (e.g. if
  the user calls IsPoint but then AsPolygon, which would be erroneous).

Check List

Have you:

• Added unit tests? Yes.

• Add cmprefimpl tests? (if appropriate?) N/A

• Updated release notes? (if appropriate?) TODO

Related Issue

• ToXYZ geometry conversion methods #420

Benchmark Results

N/A

[albertteoh]

LGTM!

[peterstace]

Thanks for the review! 😄