CoordinateSystems should be stored as fields

The original design for DataSet stored the coordinate systems as fields. The coordinate systems were really just strings that pointed to one of the point fields. For reasons that are now obsolete, the coordinate systems were moved to their own group. It would be more convenient to move them back into fields.

This should not have a big effect on most of VTK-m. There will have to be some modification to filter logic to deal with the transfer of point-fields/coordinate-systems.