read_generic_table: make lower/upper case match between data_vars and col descriptions

[FedeMPouzols]

Fixes #190

This removes the layer of ".upper()" / "lower()" in read_generic_table() and related functions. It produces data variables with names as in the MSv2 columns.

• Note this also renames read_generic_table() => load_generictable(). The functions calls from it were already renamed to load* (load_generic_cols, load_cols_into_coords_data_vars, etc.) but this read_generic_table stayed with its old "read" name as it is used in multiple places).
• This also removes some warnings in stakeholder tests/vlass.

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This contribution is made under the current ALMA software agreements.
(c) European Southern Observatory, 2024
Copyright by ESO (in the framework of the ALMA collaboration)

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