add coverage to cpp tests using cmake and gcov

You can see the coverage by compiling the tests with CMAKE_BUILD_TYPE set to Coverage.  You can do this in cmake, or with the command

    cmake ../source/ -DCMAKE_BUILD_TYPE=Coverage

You then compile the target

    make pele_tests_coverage

which compiles the tests, runs them, and outputs the coverage information in html format to a folder coverage/

The next step is to combine this with the python coverage and report both to coveralls.io