Parallel RFI filtering

[ljgray]

In its current implementation, the RFI Filter step of the daily pipeline takes ~25 minutes. Most of this time is spent taking a rolling median, and it is done in a single process. This PR should address the issue by pre-calculating a median across frequencies to flatten the auto vis. data, then distributing across the frequency axis to distribute the computation across processes.

[ljgray]

Although the products aren't quite right yet, this reduces the RFIFilter step down to ~15 seconds in a daily pipeline run

[ljgray]

Depends on radiocosmology/caput#211

[ljgray]

Interestingly, the mask being produced is different from that produced by the previous method.

[ljgray]

@jrs65 this produces the same mask now as previous revisions, so it should be good to go unless there are additional changes.