Support any defined Magnitude Scaling Relationship class

[matley]

We want to support any defined magnitude scaling relationship (MSR), i.e. any derived class of BaseMSR (https://github.com/gem/oq-hazardlib/blob/master/openquake/hazardlib/scalerel/base.py#L63) to be used in the engine.

Currently, the list of supported MSR is hardcoded in the implementation (https://github.com/gem/oq-engine/blob/master/openquake/engine/input/source.py#L43).

A more future proof solution is get them at runtime by using the MSR registry (see https://github.com/gem/oq-hazardlib/blob/master/openquake/hazardlib/scalerel/__init__.py#L52).

[matley (matley)]

https://github.com/gem/oq-engine/pull/1280