Consolidate MS support into xds_{from,to}_ms

[sjperkins]

• dask-ms version: 0.2.15
• Python version: 3.8
• Operating System: Ubuntu 20.04

In #272, @caseyjlaw tried to use xds_from_ms to open a sub-table of the MS. This didn't work because xds_from_ms adds some default values (grouping and indexing columns) for opening the MAIN table of an MS. Due to this, we currently use xds_from_table to access sub-tables in our applications.

However, it occurs to me that @caseyjlaw was trying to do the most obvious thing, given the affordance. It could be that split is unnecessary and confusing and that xds_{from,to}_ms should be the interface for dealing with all Measurement Set-like datastores (CASA/zarr/parquet), as well as their sub-tables.

Then, xds_{from,to}_table can be used for general undefined table types, and we could define xds_{from,to}_xxx methods to handle some xxx specification (calibration methods for example).

Any opinions on this @JSKenyon, @landmanbester, @o-smirnov ?

[JSKenyon]

I had had a similar thought and I am all for it. This would likely be a breaking change, but I think it would be worth it. It means that we can abandon MS-like syntax in xds_{to,from}_table (i.e. don't introduce a MAIN table for no reason) and remove the unintuitive double colon syntax that it necessitated. This functionality can be retained/added in xds_{to,from}_ms to make it "equivalent" with MS access via python-casacore. I think that xds_{to,from}_table should be sufficient for all other use-cases.