Adds source models writer to nrml

Adds the utilities for writing the source model

1) Adds method to every mtk source class to convert to corresponding class in openquake.nrmllib.models
2) Adds a general conversion function to allow for certain user defined behaviours (e.g. basic error checks, acceptance of defaults etc.)
3) Adds simple method to hmtk.sources.source_model.mtkSourceModel class to call the openquake.nrmllib.hazard.writers.SourceModelXMLWriter