Fix MPICH vs. MPICH2 inconsistencies

An MPICH 3.x-based toolchain exists for both ClangGCC and GCC, however, the first uses MPICH as MPI toolchain name and the second MPICH2. Since the project has renamed itself back to MPICH with version 3.0 (see http://www.mpich.org/about/overview/ last paragraph), MPICH seems to be the right name to use. This PR fixes these inconsistencies.

Note: This PR requires changes in the EasyBuild framework (see https://github.com/hpcugent/easybuild-framework/pull/1101).