OpenFOAM EasyBlock cleanup and update

This a updated version of the OpenFOAM EasyBlock.
It goes together with PR https://github.com/hpcugent/easybuild-easyconfigs/pull/957 for the OpenFOAM EasyConfig and patch files.

The main changes are:
- additional sanity checks, especially for the MPI-based version of some libraries (Pstream, ptscotchDecomp)
- use the same compilation method for all the MPI toolchains, based on `mpicc`, `mpicxx` so we don't have to explicitly give the list of the MPI libraries
- use `EASYBUILDMPI` instead of `EASYBUILD` as a value for `WM_MPLIB` environment variable because OpenFOAM build system will only build the MPI-based Pstream library if this value contains `MPI` (cf `src/Pstream/Allwmake`)

It has been tested and seems to build correctly for `OpenFOAM` (2.0.1, 2.1.1, 2.2.0, 2.3.0) built with `goolf` (1.4.10) and `ictce` (5.3.0). `OpenFOAM-Extend` and `iqacml` toolchain have not been tested.

Please review and let me know if you have any comment so I can continue the same way for OpenFOAM-Extend.