Multi elpa

Provide both elpa~openmp and elpa+openmp in the same toolchain. Octopus needs the corresponding one depending on whether it is compiled without/with openmp support.

In the past we had modified the elpa package to install both variants under the same hash. Re-applying that commit lead to problems with elpa-dependent libraries. With the proposed changes elpa~openmp and elpa+openmp stay independent. Name clashes are avoided via the module adjustments. We have to restrict to unify:when_possible. We would notice multiple variants of other libraries via name clashes during the module file generation so that should be okay.