Build internal muparser more simply
We now literally bundle muparser 2.3.2 and use its build system with minor changes. Those are to not build the samples, to use the value of GMX_OPENMP to set whether OpenMP is use, and use target_include_directories to propagate include directories properly.
The simple muparser tests are now run as part of the GROMACS tests. They are very fast.
Refs #4165 (closed)