FIX: finalise our class names - make sure we're happy they fit well alongside other well known DSL - especially GEM's NRML XML

[chrisbc]

@chrisdicaprio the one outstanding change here is the potential rename of FaultSystemLogicTree class to a more generic BranchSet (similar to NRML). A good reason for this is when when create hazard calculations with the combined permutations spanning multiple fault systems.

The implementation is in nzshm_model.source_logic_tree.version1.logic_tree.

Because this will impact the dataclass serialisations, I suggest we decide this before we migrate any more history SLT version into the model.

[chrisdicaprio]

@chrisbc are you proposing changes to source logic tree v1 or just v2?