Add functionality to CDDS/MIP Convert to appropriately handle region

For the initial submission to CMIP7 we will not submit regional subsets (e.g. variables over 30S-90S), but there are some ocnbgchem variables which are only requested over this region.

We should
1. work out how best to handle specifying the region for a variable
2. work out what grid labels should be applied
3. work out how to collapse 2D lat and lon into 1D for ocean variables.

Start by tracing the CDDS/MIP Convert path that handles variable regions and grid labels; the payload does not name specific files or tests. Review the ocnbgchem variables that require regional coverage and determine how region specification, grid labels, and 2D ocean latitude/longitude collapsing should work. Done means these behaviors are defined and implemented for the CMIP7 submission constraints.

Written by the indexing model from the issue text.