Add component OpenMP for MUMPS to compile with multithreading
I had an issue when compiling MaPHyS with multithreading + MUMPS on OCCIGEN: impossible to find MUMPS. The MUMPS test could not find (and link) the OpenMP library. I added OpenMP as a MUMPS component so that FindMUMPS could use a FindOpenMP and link correctly the MUMPS test. It worked fine to compile MaPHyS.