io.shapefile: add support for writing origin uncertainties to shapefile

[megies]

This PR adds functionality to write event origin uncertainties to shapefile format (e.g. to use in GIS applications).

ToDo:

• example / tutorial
• test
• write uncertainties into shapefile table for reuse in GIS applications
• other information on the event would be useful to have in the shapefile database table as well, especially origin quality metrics, e.g. number of stations used in location, azimuthal gap, ...

+DOCS

Usage example:

from obspy import read_events
cat = read_events('my_catalog.xml')
cat.write('/tmp/tmp_my_events.shp', format='SHAPEFILE',
          error_ellipses={'filename': '/tmp/tmp_my_events_errors.shp', 'epsg': 31468})

Potential improvement:

• also write origin uncertainties to feature tables (so that filtering by error margins in GIS applications is possible)
• automatic lookup of appropriate UTM zone, based on event locations in catalog (or raise exception if catalog is too wide-spread so that a single geocentric zone will not work)

[megies]

Maybe should also add writing columns with hypocenter uncertainties to the shapefile, for use in GIS applications..

Done.

[megies]

Closing this one. The whole approach was a bit awkward anyway and uncertainties are now in the main shapefile table, so applications can use that information for plotting.

[megies]

Mada a backup of this branch here: https://github.com/megies/obspy/tree/shapefile_error_ellipses