fix hierarchical mns w.r.t. MPI installed with a dummy toolchain

enhanced version of #983 by @kcgthb (which should have been opened targeting `develop`
In a hierarchical scheme, it is. You need to specify a compiler to be used in combination with this MPI library, even though the installation itself may not need it (cfr. Intel MPI).

In the 'standard' hierarchy, an MPI is always on level 1, where level 0 is core modules usually corresponding to compilers. You need to specify a particular compiler, to specify in which "branch" the MPI library should be installed.

For now, this will result in duplicatie installations when the same MPI library needs to be available for different (versions of) compilers, but we can enhance the framework such that multiple modules can be generated for a single installation (I hope to tackle that in #1018).