add nonstco
tests improved
Merge pull request #91 from matley/nonstco

Added non-structural cost attributes and elements
hazard/writers:

Added `SourceModelXMLWriter`. It still needs documentation, though.
hazard/parsers:

Fixed a comment.
tests/hazard/writers_test:

Added `SourceModelXMLWriterTestCase`.
tests/hazard/writers_test:

PEP8 whitespace fix.