Adds Rupture Mechanism class

The generation of the 3D rupture planes for characterising the finite rupture distances depends on the rupture mechanism (strike, dip and rake). As this information may be absent or incomplete, the mechanism itself is defined as a probability mass function, with multiple possibles mechanisms and their corresponding probabilities, and this is subsequently used to generate the probability density of source-to-site distances within the shakemap calculation. This MR implements an object for rupture mechanism distribution from no data, incomplete data and complete data.