Make fitscheck check compliance by default

The ``fitscheck`` command only checks checksums by default, but has a ``--compliance`` flag to check for compliance with the FITS standard. As a user I had assumed for several years that ``fitscheck`` defaulted to doing compliance checking too. I think we should actually just turn on compliance checking by default and add a flag to disable it, since I think other users will likely be expecting the same.

Start by locating the fitscheck command entry point and reading how checksum checking and the --compliance flag are implemented. Inspect any nearby command tests if available; done means compliance checking is enabled by default and a flag can disable it while checksum behavior remains covered.

Written by the indexing model from the issue text.