Fix multigrid preconditioner

Description

The problem was that the multigrid preconditioner used the general initialization routine, leading to potentially less than 3 mesh levels for core counts larger than 7, but then 3 levels were accessed, leading to a segmentation fault. This fix now uses a different object for the multigrid of the preconditioner and ensures that this has always 3 levels.

News snippet

Fix multigrid preconditioner

Checklist

• I have checked that my code follows the Octopus coding standards