✨ Add support for multiple polarizations in SIGPROC filterbank files.

[astrogewgaw]

While typically we encounter SIGPROC filterbank files with only Stokes' $I$ data, they support storing full polarization data as well (that is, all Stokes' parameters: $I, Q, U, V$). I should think about adding support for that as well, if possible. IDeally, the code for reading filterbank files should be generic w.r.t the number of polarization in the data.