Resolve "Bug in states_elec_generate_random()"

Description

There is a bug in states_elec_generate_random, which prevents the consistent random wave functions between serial and parallel runs. The problem is that the random seed was not consistently shifted in order to account for random draws on other processors. This is fixes now by introducing a second (post-) shift operation.

News snippet

This MR fixes a bug in states_elec_generate_random, which prevented consistent random wave functions between serial and parallel runs.

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.

Closes #315 (closed)