Add softcore ring-breaking nonbonded force

[lohedges]

This PR adds support for softcore ring-breaking and ring-making forces to smooth the onset of nonbonded forces when the ring topology changes, i.e. we don't immediately go from zero to full nonbonded, which can introduce a large repulsion energy. The code follows the same approach as the ghost14 force field, only for the atoms involved in the ring-breaking bond. The required atoms are flagged during the BioSimSpace merge code and a unit test has been added to somd2 to ensure that the new forces are present when they should be and are active/inactive at the correct points in the schedule. Putting these tests in somd2 makes sure that we are testing the producing ring-breaking schedules, e.g. we don't have to duplicate and synchronise them here.

Testing on several complex perturbations has indicated that the new forces improve stability and overlap in the vicinity of the nonbonded onset. The major contributor to poor overlap is still the large conformational change (and resulting nonbonded energy change) as the ring topology adjusts, but these additions help as a stability safety net when the bond atoms get too close.

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• I confirm that I have added a test for any new functionality in this pull request: [y]
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[lohedges]

I've realised that I will also need to exclude one of the atoms in the bond pair from the GhostNonGhostForce if it is a ghost in one end state. This could happen in a perturbation where you add a dummy chain at one state, then bond it to an existing ring. As such, the atom at the end of the chain would also a ghost.

[lohedges]

Actually, easier the other way round, i.e. don't create this force if a bond atom is a ghost.