Broke out each type of source into separate files.

I already sketched out a few of the changes to the Area Source.
Updated test run script to recursively search the examples/ dir for files to validate.
Sketch of new-and-improved source model structure
Simplified source models, removed unnecessary gml:ids.
Relaxed restrictions on tectonic region type definitions (plain strings are allowed now).
Updated nrml schema version references.
Missed disaggregation in the last commit.
Loosened restrictions on disagg tectonic region type.
Cleanup of area-source.xml and point-source.xml example files.
Removed the include for the old source_model schema. This is going to be removed later on in this branch.
Added the PositiveNonZeroDoubleType to restrict float numbers to the range (0.0, inf).
Stubbed out the new source model schema files.

Started populating area_source.xsd with schema defns plus some notes.
s/NonNegativeDoubleType/PositiveDoubleType/g

This name is better.
s/NonNegativeIntegerType/PositiveIntegerType/g

A more consistent name.
Area source now validates with schema up to and including <ruptAspectRatio>.
Renamed NonNegativeDoubleList to PostiveDoubleList.
Corrected a typo
Finished schema for area sources. Next, point sources.

At this point, I can start thinking about trying to abstract the common bits.
Completed schema definition for point sources.
simple-fault-source.xml: Fixed a minor syntax error.