Several improvements and fixes for pointing_xds + option to specify chunk sizes in GB

[FedeMPouzols]

This fixes and resolves #128 (fixes and new features for pointing_xds) and resolves #130 .

Note that I merged all the branches from the individual tasks of those two issues into the last one. That means that the 5 features/fixes related to the pointing_xds + the feature of chunksizes in GBs are all included in this PR, for convenience. I could also break this PR into individual incremental PRs (that would be 6), which would make reviewing easier.

Note well:

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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