Add `MassMatrixDiagonalPreconditioner`

Solves the following issue(s):

Closes #257 (closed)

Core changes:

Add a new preconditioner MassMatrixDiagonalPreconditioner which relies on the methodoloy described here.

Uncomment the diagonal method for StencilMatrixFreeMassOperator.

New psydac version to use sqrt flag on diagonal method.

Model-specific changes:

None

Documentation changes:

None