MappingProxyType breaks in-place filtering

[gtca]

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This commit 5ccf921 made .obsmap / .varmap fields to be of type MappingProxyType, which broke the in-place filtering implemented e.g. in muon:

FAILED tests/test_muon_preproc.py::TestInPlaceFiltering::test_filter_obs_mdata - TypeError: 'mappingproxy' object does not support item assignment

It might be enough to implement @obsmap.setter and @varmap.setter to fix it.

[ilia-kats]

Ah, sorry about that. I was trying to prevent people from shooting themselves in the foot by accidentally modifying these, since we're heavily relying on the maps internall and they're difficult to get right, but I guess a different approach is required. Perhaps a setting?

I will fix it