Enable GS calculation if one electronic system is present and use it in a test

Description

Enable the ground state in multisystem input files, only if the Systems block contains purely electronic systems (for the moment).

The only remaining issue is that the stdout has very little information of what is going on, it would be good to add some more information in the future.

Solves #811 (closed)

Checklist

• I have checked that my code follows the Octopus coding standards
• I have added tests for all the new features added in this request.