Add OpenQBMM

As per discussions with @andy and @albertop - aim to add OpenQBMM in as a submodule for 1912.

Alberto has rejiged some bits to compile will 1906 (only some deprecation warnings for dimensionedType that can be safely ignored). I've recompiled his branch with the 1912 develop and nothing came up with gcc-7.4. May need to recheck with gcc-4.8.5, but probably not too bad.

As soon as we get a mostly finalized branch (even better, tagged) can add the submodule.

Q: The subdirectory name as under modules/OpenQBMM, modules/open-qbmm ...? We are flexible.