Make UVFlag add smarter

[adampbeardsley]

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Right now the UVFlag add method requires an axis to be specified and it blindly concatenates. We should make it smarter to check for compatibility and figure out which axis to add along, like UVData. Similar to UVData, we should also add a fast_concat method.

[mkolopanis]

The current implementation of add basically is fast_concat luckily. Just need to build up checks for a more safe add.

[bhazelton]

Generally need to add more checking as is done with the other objects. In particular, make sure the right thing happens in the case of overlapping and differently sorted data.

see https://github.com/RadioAstronomySoftwareGroup/pyuvdata/issues/1102 and https://github.com/RadioAstronomySoftwareGroup/pyuvdata/pull/1105 for reference