hazard/writers:

Abstracted BaseCurveXMLWriter, to be extended by HazardCurveXMLWriter and
UHSXMLWriter.
hazard/writers:

Sketched out UHSXMLWriter. The constructor is complete (with a few validation
checks), but the `serialize` method needs to be implemented.
tests/hazard/writers_test:

Added validation tests for UHSXMLWriter and stubbed out 3 serialization tests.
tests/hazard/writers_test:

Added `UHSData` mock object for testing serialization.