CTest: Rename coverage implementation for "Python" to "Cobertura"

The coverage.py tool writes out an XML that conforms to the Cobertura
Coverage tool standard.  Rename the cmParsePythonCoverage files to
instead be cmParseCoberturaCoverage.
CTest: Generalize Cobertura coverage format handling

Add support for Cobertura coverage files written by Java.

Add a test which uses the report from a Java run of Cobertura to calculate coverage.

In the documentation of CTEST_COVERAGE_COMMAND, give a sample .sh file to merge
the Cobertura .ser files and generate the XML report from the merged file.
Merge topic 'expand_cobertura_coverage' into next

a7b8f4e7 CTest: Generalize Cobertura coverage format handling
e33d0d0b CTest: Rename coverage implementation for "Python" to "Cobertura"