Enable flagging for tod stats

[earosenberg]

In #1028 we added summary statistics for tod, subscans, and PSDs, computed along the samps (or nusamps) axis. A boolean array or slice can be passed to flags.get_stats (link) to mask the signal array before computing the statistics. This was added to enable limiting the frequency range if the signal is a PSD.

Currently the mask is limited to 1d samps/nusamps, and cannot be used with subscan=True. It would be nice to

• Allow mask to be a Ranges
• Allow mask to be 2d / RangesMatrix
• Combine the mask with subscan_flags to allow masking and subscan computation together.