Add ISO schemas and modules for GMI, GMD, and GCO

The GMI schemas are not in the OGC schemas module, therefore the new ``52n-isotc211-schemas`` module was added. The GMD and GCO modules were created based on the schemas in the module ``52n-ogc-schemas`` and therefore follow the date-based versioning.

Alternatively, GMD and GCO could be added from ISO/TC 211 repo as well (http://www.isotc211.org/2005/) and then get the version from these schemas, which is ``0.1``).