UVFlag: support applying time-, frequency-, and baseline-dependent flags to UVData objects

[PyxieLouStar]

It would be helpful to have an integration between UVData and UVFlag to easily apply time-, frequency-, and/or baseline-dependent flags to a UVData object.

One option would be writing a function in UVData that initializes a UVFlag object from the UVData object, calls a utility UVFlag function which flags the desired selections, and then applies the flags back to the UVData object with apply_uvflag.

[PyxieLouStar]

I am hoping for some discussion of the best path forward, and am happy to work on implementing whatever UVFlag and UVData functions make sense.