This variable is set to allow the use of the nice ParseAndAddCatchTests script
in the case where a launcher is needed to execute the script.
This is introduced to allow to launch unit tests using mpi. In this case one can
write for instance
set(OptionalCatchTestLauncher ${MPIEXEC} ${MPIEXEC_NUMPROC_FLAG} ${NUMPROC})
before calling the ParseAndAddCatchTests function.
If source files are defined using relative paths, CMake will compile the tests, but this script will (sometimes) fail to find and parse the tests from the source files. I have added an explicit warning when ParseAndAddCatchTests fails to find a source file.
Added cmake script to parse the source files containing Catch's test and generate ctest definitions.
It generates one ctest test per `TEST_CASE` and labels them, using the `TEST_CASE`'s tags.
Closes#719.
