include a new "--extract-geometry-only" option and tests

The idea of this feature is enable some modeller to extract only the geometry from very simple _shapefiles_ (e.g. areas, simple fault traces and points).

Adding this option, some _default_ parameters (MFD, upper and lower limits, NPD, HDD) is written to the NRML source model file. Of course, it should not be used to some hazard calculation, but as input to HMTK modelling.