Add parameter for Polcal spatial binning (to speed up tests)

With ViSP 2.0 we hardcoded the number of spectral and spatial bins used during polcal to be 1 and all of them (2560), respectively. That works great, but running polcal fits for all 2560 spatial bins takes a long time and it has really slowed down our deployment machinery (due to long recipe processing tests).

This PR parameterizes the number of spatial bins so that we can set it to a small number during tests. The expectation is that we’d still fit all spatial pixels on PROD, but now we can fit fewer bins during RP runs.

That said, this change is scientifically valid and tests have shown that moderate amounts of binning don’t change the results to any large degree.