Further SOMD1 compatibility updates

[lohedges]

This PR adds the following minor updates for SOMD1 compatibility and testing:

• Check for light atoms in perturbable molecules by the maximum end-state mass only.

• Check that the constraints are the same in both end states and log a message if not.

• I also added a missing constraint kwarg to the minimisation.

[lohedges]

These fixes require the fix_constraints branch of sire.

[chryswoods]

I'm happy to merge fix_constraints in if you want? I left it until we were happy we'd fixed most of the bugs.

[lohedges]

Yes, no problem. I assumed that I'd figure out the rest of the problem quickly and add to the branch. Alas.

[lohedges]

I've added a few further tweaks:

• End-state constraints are now checked for all perturbable molecules in the system.
• Make sure that pressure can be disabled from the command-line, i.e. add special handling for the case of "None" .
• Exposed the barostat_frequency kwarg via the Config class.

[mb2055]

Looks good 👍 all working on my end