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.

Most probably, the OpenFOAM-extend patches must be updated in the same manner as I did for OpenFOAM.
I cannot look in details right now, but here are a few clues.

About Pstream:
It might be that OpenFOAM-extend does not build the `mpi/libptscotchDecomp.so` for some reason. The build of the Pstream libs should be triggered in `src/Pstream/Allwmake` by matching the value of the `WM_LIB` variable. The key part of the patch for this is for file `etc/config/settings.sh` with add the following lines
```
+EASYBUILDMPI)
+ export FOAM_MPI=mpi
+ ;;
+
```

About SCOTCH libraries:
- You should probably look at the patch for `etc/config/scotch.sh` that does
```
-export SCOTCH_VERSION=scotch_6.0.0
-export SCOTCH_ARCH_PATH=$WM_THIRD_PARTY_DIR/platforms/$WM_ARCH$WM_COMPILER/$SCOTCH_VERSION
+if [ -n "${SCOTCH_ROOT}" ] ; then
+ export SCOTCH_ARCH_PATH="${SCOTCH_ROOT}"
+else
+ export SCOTCH_VERSION=scotch_6.0.0
+ export SCOTCH_ARCH_PATH=$WM_THIRD_PARTY_DIR/platforms/$WM_ARCH$WM_COMPILER/$SCOTCH_VERSION
+fi
```



I hope this will help.