MonotoneMGStep inherits from MultigridStep

... though it is not a LinearIterationStep.

This also caused trouble with a7e17300 (reverted in a2547fe8) because the MonotoneMGStep modifies the transfer operators.

If there is a huge overlap of code for MonotoneMGStep and MultigridStep, in particular to provide all the mg members and their setters and getters, maybe a separate base class is a convenient solution.