fix HierarchicalMNS for cgoolf and goolfc toolchains + add iompi definition

This enables making HierarchicalMNS compatible with the `goolfc`, `cgoolf` and `iomkl` toolchain (see also https://github.com/hpcugent/easybuild-easyconfigs/pull/1099 w.r.t. `iomkl`)
and deal with the `KeyError` :smile: