Add WCS DescribeCoverage parsing for WCS 1.0.0 and 1.1.0

Adds WCS DescribeCoverage parsing, versions 1.0.0 and 1.1.0.  This pull request includes prerequisite work submitted as part of other pull requests, including fixes to GML and GetCapabilities parsing.  Processing those pull requests first will reduce the complexity of this one.

Includes full suite of tests; all tests passing.