hazard/writers:

Corrected copyright date in license header.
tests/hazard/writers_test:

Corrected copyright date in license header.
hazard/writers:

Implemented `serialize` method of UHSXMLWriter.
tests/hazard/writers_test:

Implemented serialization tests for UHSXMLWriter. Includes cases for:
    * logic tree end branch
    * mean
    * quantile