Coordinate systems are stored as Fields

Previously, DataSet managed CoordinateSystems separately from Fields. However, a CoordinateSystem is really just a Field with some special attributes. Thus, coordiante systems are now just listed along with the rest of the fields, and the coordinate systems are simply strings that point back to the appropriate field. (This was actually the original concept for DataSet, but the coordinate systems were separated from fields for some now obsolete reasons.)

Fixes #734 (closed)