Implements the Magnitude Scaling Relation of Bonilla et al., (1984)

This PR implements a magnitude scaling relation (MSR) defined by Bonilla et al., 1984. This MSR is useful for older PSHA calculations, and can be found in models such as that of the Philippines (Tenhaus et al., 1990).

The MSR is a magnitude-length scaling relation, rather than the area scaling currently supported by OpenQuake. Currently this is not directly compatible, so we opt to define multiple implementations, each assuming a different rupture aspect ratio. This is a temporary solution to enable implementation of this particular MSR. An overhaul of the scaling relations to support the possibility of defining length, width and displacement scaling will be needed in the future.
Having all this classes is really really ugly. Can't we have an __init__ in the GSIM classes and instantiate different objects for different values of the aspect ratio? I understand that this will have a (large) impact, but the proliferation of classes is terrible and I know that this is not the only place where we have set of classes.