add horizCoordSysName option for modis sinusoidal

[srearl]

Hi @mbjones and @mobb,

I need to publish a large number of MODIS-derived rasters, but the projection of the raster (MODIS Sinusoidal, see here and here for refs) is not included among the list of horizCoordSysName options. Could we add that as an EML-valid option?

Separate but related, I have been working with a lot of spatial data lately and find the intuitive yet arbitrary names of the coordinate systems that we use in EML to be somewhat frustrating. I realize my modis issue above is a bit of an odd case, but it seems a better approach generally than using arbitrary names in the schema would be to use instead epsg, sr-org, etc. codes - a plus if we could link those to a spatialreference.org or related API. Ah, maybe that could be the next realm for annotations! Sorry that I am thinking of this so late, maybe something to discuss for version EML v3.

[mbjones]

Implemented in PR #353, awaiting merge to a release branch.