Improvements of the multigrid Poisson solver

Description

Minor improvements of the multigrid Poisson solver

• Add some documentation
• Only iterate one time before computing the residual, instead of 5 times (pre+post smoothing steps)
• Implement W-shape multigrid Poisson solver. An option is added and a unit test is added.
• The weight for the Jacobi relaxation is now not used for the Gauss-Seidel method. The option is renamed for clarity.

Changes to the multigrid: The injection and restriction operations are now self-adjoint operations

• As the order is now fixed at linear interpolation, we do not need the option for setting the order in the unit test.

The Gauss-Seidel relaxation is now fixed for curvilinear coordinates.

Closes !1353

News snippet

Improvements of the multigrid Poisson solver and the multigrid method in general.

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.