[NEW FEATURE] Local position in the validation catalogue for MCCD

Hello,
I have noticed that in the MCCD catalogue the x and y positions are given on the global grid that MCCD is using for the multi-CCD approach. Due to this, the MCCD package is required to read the catalogue and transform back the positions to a local reference mining x and y on each CCDs. I think it could be nice to have the local positions directly in the catalogue to avoid the call to MCCD. That would simplify the validation a lot because the catalogue will follow the same format as PSFEx and we could have only one validation function instead of two. I think that could also simplify the comprehension of the catalogue by external users.

I see two ways to handle this:

1. This can be done in the merge_starcat module but I found that a bit sub-optimal because we will have one function for MCCD catalogues and one for PSFEx (like it is right now).
2. Do it in MCCD when it does the interpolation. Like that we could also simplify the merge_starcat module. What I would propose is just to add one column for the LOCAL_POS along side the GLOBAL_POS so the user can choose which one to. It will not break the pipeline as it is and give time to update the different modules that are impacted.

One solution is only in ShapePipe the other is in MCCD. I have a preference for the second one. Let me know what you think 🙂
(Maybe this issue should be moved to the MCCD repo, let me know)