Casida: use scalapack or elpa for solving eigenproblem in parallel

Description

Add possibility to use scalapack or elpa for solving the eigenproblem. This is needed for large systems because the computational cost of the eigenproblem scales with the third power of the number of electron-hole pairs.

News snippet

Make use of scalapack or elpa possible for Casida

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.