Command to generate Proj fields

[cholmes]

Having the proj fields in catalogs is pretty important, and the information is usually easily obtainable from the assets, so it'd be nice if there was a command that would generate the main proj fields from the data. My first crack of what those would be are epsg if available, fallback to proj wkt or proj json, plus shape and transform.

This could be a dedicated command like 'move assets', and/or could be an option during a copy command.

[gadomski]

This is complementary to #179, as the auto-generation of projection fields would be part of a naïve create-item command.