Fixes in schema checker/doc to sync with spreadsheet

[FedeMPouzols]

This is to add a few small fixes still needed in the schema to match the spreadsheet.

For now I see:

• a discrepancy in pointing_xds: there was some confusion of time_pointing with an old leftover time_polynomial and the n_polynomial.
• field_and_source_xds: clarify what is the intended use of the "ZD". It is being used inFIELD_PHASE_CENTER and it seems that it should use in a similar way in the other data vars that do not have other dimensions than time/ephemeris (or not listed in its Data[... definition but included in the ...Array class used from there). Also, in these Data[... definitions, enforce QantityArray (or LocationArray wherever appropriate) in several data_vars, rather than an array of raw floats.
• Also in field_and_source_xds: FIELD_DELAY_CENTER is stricken through but it was still in the schema. Update another variable name: OBSERVATION_POSITION=>OBSERVER_POSITION.
• partition_info: descriptions were missing.
• polarization_mixed optional coordinate was missing.
• earth_location is deprecated, but there are leftovers of it and the ellipsoid attribute in antenna_xds (earth_location_offset), field_and_source_xds, and _utils/schema.py.
• Discrepancies in antenna_xds: BEAM_OFFSET and (ANTENNA_)FEED_OFFSET should be removed from conversion. FOCUS_LENGTH and RECEPTOR_ANGLE are missing the ANTENNA_ prefix.
• doppler_shift_velocity attr of field_and_source_xds is still in uppercase and not checked as quantity.
• time_weather is not yet included in schema

After #256 has been merged:

• VisibilityXds => CorrelatedDataXds + various renaming of vis, read/open, etc. in .rst (vis_schema.rst, vis_api) files. Also rename "visibility->correlated data" in text, headings, file names, etc.