Refactor *_omp mex routines to use clibgen

[mducle]

In anticipation of migrating to Python, the current mex codes should be translated to be a plain C function or C++ class which can be interfaced using loadlibrary/calllib (for C code) or clibgen (C++).

This will then allow easy binding to Python using either CPython directly (for C code) or pybind11 (C++)