Generate merged code coverage reports with pycobertura

Cobertura is a coverage tool for Java that outputs reports in XML. The tools that we use for Python and C coverage respectively, coverage.py and gcovr, can both output the same format. So we can use tools for Cobertura to generate merged, multilingual coverage reports.

This would be a lot prettier if we could use coveralls or codecov.io, which support multilingual test coverage. However, those products don't integrate with git.ligo.org (or at least, they don't integrate for free).