Add flexibility in GCData manipulation - recent merged PR causes failure when trying to vstack 2 GCData tables

[combet]

I used to be able to vstack (from astropy) 2 GCData galaxy catalogs, but after updating my CLMM version to the latest main branch (1.16.7), doing:

from astropy.table import vstack
galcat_all = vstack([gc_object1.galcat, gc_object2.galcat])

gives the following error

ValueError: coordinate_system must be changed via update_coordinate_system method

I guess this is related to the added protection that was added to the metadata of GCData in a recent PR. I think being able to merge 2 galcat tables is a useful functionality to preserve. @caioolivv , @m-aguena

[m-aguena]

@combet , that is a fair point. @caioolivv could you take a look at it?

[caioolivv]

Yep, already have a good guess of how to fix this (basically hijack some of Table's internal functions like we discussed in the previous coordinate system PR, but that will cause some maintaining burden).