Adds lightweight PSHA calculator to hazardlib

This PR adds the functionality to run PSHA calculations from the hmtk. Methods are added to each source model class to convert to a full openquake-hazardlib source model. A light-weight PSHA calculator is added.

Initially the tools were intended to include options for parallelisation of the hazard curves. After extensive testing and exploring I reached the conclusion that it is not currently possible to parallelise the hazardlib in the way intended here due to the limitations (and as yet poorly understood behaviour) of the multiprocessing library. A parallelisation by source option is included for (hopefully) future compatibility, but the function is not parallelised at present.