Patch elpa

[iamashwin99]

Waiting for MR
#91

Elpa variant isnt properly set in the octopus in spack package.
Steps to fix:

• enable elpa only when +scalapack
• Update tests to test elpa and scalapack only when mpi is selected
• Set FCFLAGS_ELPA
• Include patch to the m4 for elpa_openmp detection

Fixes #66

Closes #66

[fangohr]

From the discussion today (with @iamashwin99 @glaweh @lang-m):

• When the Octopus package for spack switches to cmake, this merge request is not needed anymore.
• Is cmake build system completed: can we compile Octopus with cmake through spack-compiled depedencies?
• Is the autoconf-based configuration in Octocus working for elpa (mostly out of the box) in version 14.0 of Octopus? If so, support this.