Adding support for flexible spws with UVFlag objects

[kartographer]

Following the merge of #909, UVFlag objects and methods now need to be updated to allow for the new handling of multiple spectral windows, and more generally to allow for the proper handling of the flexible spectral window feature found in UVData.

[jsdillon]

In particular, I need a .use_future_array_shapes() method in UVFlag in order to update hera_qm to eliminate the spw dimension (https://github.com/HERA-Team/hera_qm/pull/413), which in turn needs to be finished to do the same thing for hera_cal (https://github.com/HERA-Team/hera_cal/pull/797)