Fix bug in restriction

The restricted quantities (residual -> RHS) are cell integrals, not point values, so they should be restricted accordingly. Eventually this should be fixed in the firedrake restriction operator, but a quick workaround, just multiply the coarse grid quantity by a factor of four.
Adapt parameters

Changed parameters in main program for better convergence.
Write sphinx warnings to a file

Change the makefile options to ensure that sphinx writes warning messages to a file, this simplifies debugging.
Use maximal eigenalue in smoother

Instead of using the full diagonal matrix D in the smoother it is now possible to use an upper spectral bound obtained by replacing D by the identity matrix multiplied with the largest eigenvalue of D. This makes the smoother symmetric, which is not necessarily the case for a more general diagonal D.
Main program for calculating eigenvalues

This code calculates the eigenvalues of the operator and the smoother iteration, which might be useful for testing and debugging.
Ignore sphinx warnings file