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.
I think that there is an easy solution. I can extend the GMPE logic tree parser to understand arguments to the GMPE classes. For instance you could write

 <uncertaintyModel>BonillaEtAl1984(7.0)</uncertaintyModel>

instead of 

 <uncertaintyModel>BonillaEtAl1984Aspect7p0</uncertaintyModel>

and you would need to define only one class in hazardlib. Then the code in commonlib which is reading the GMPE logic tree would return GMPE instances with the right arguments. This is easy and same to implement if the arguments are literal (i.e. floats, integers, strings, ...)
which is the case here.