Implements initial complete shakemap feature

Implements the initial Shakemap class that combines the earthquake, site and ground motion models (here defined in terms of a dictionary containing a list of models and a list of their respective weights). This MR addresses two issues:

1. The migration of the main shakemap calculations from the original shakyground repository (#10 (closed))

2. The implementation of the synthetic rupture generator to handle the construction rupture surface needed for the ground motion calculations given the limited available information on the earthquake source (#9 (closed))

Additional methods have been added to the Earthquake, SiteModel and RuptureMechanism classes to handle the synthetic rupture generation and subsequent shakemaps more cleanly.