3d faults

This pull request adds the capability for the source_model_converter.py to generate 3D fault planes for the simple fault sources included in a NRML file. I didn't add tests so far. This functionality is not altering the behaviour of this converter since the generated shapefile is not used for the creation of NRML.

I run source_model_converter.py with a NRML representing a large regional model and the results looks okay from a visual point of view.