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.

Other than a few details, it uses the exact same build procedure. The project was forked from the original OpenFOAM some time ago, and they have been patching the codebase, but the build procedure is mostly still the same.

I don't think the missing files are installed at all. Note that the `dummy` `libPstream.so` is there, but the `mpi` one isn't.

The missing scotch libraries probably have to do with it simply not finding or using SCOTCH, which is one of the problems you've fixed for OpenFOAM.

I'll try and take another look at this later.