Improve inferred MS name in absence of --output-ms
Improve inferred MS name in absence of --output-ms

The default MS name got a bit creaky once RDB files are downloaded via
URL (and with tokens). Rework the formula to do the following:

  - Strip off all host names, schemes and query strings (i.e. URL-safe)
  - Strip off all directories so that the MS goes into working directory
    (as promised in the code comment but not actually done)
  - Get rid of the ".full" part of RDB filename (it's the same dataset)
  - Only add frequency info if the dataset has multiple spectral windows
  - Add ".et_al" if we are concatenating multiple datasets

This addresses JIRA ticket SR-1828.

ska-sa/katdal