Implement NonParametricSource in NRML

We need to provide support for the Non Parametric Source in NRML (see https://github.com/gem/oq-hazardlib/blob/master/openquake/hazardlib/source/non_parametric.py#L30)

A NonParametricSource is a seismic source which is defined by the following three base parameters:

- source ID
- source name
- tectonic region type

Earthquake ruptures associated with the source are explicitly defined through a list of tuples, each tuple containing:

- probability mass function for the number of rupture occurrences (list of tuples, where the first value is a probability level and second value is number of occurrences)
- earthquake rupture characterized in terms of the following parameters:

     - magnitude
     - rake angle
     - hypocenter (longitude, latitude, depth)
      - rupture surface

The rupture surface can be defined as:
- collection of planar surfaces (1 or more)
- simple fault geometry
- complex fault geometry

This is similar to the possibility given for the CharacteristicSource (see https://github.com/gem/oq-nrmllib/blob/master/openquake/nrmllib/schema/hazard/source_model/characteristic_source.xsd#L29)