More things to be done

# Bugs

# ToDo

## Bugs

These issues are not real bugs, but unexpected behaviour that might

lead to serious trouble if you're not aware of.

* ProjectedBlockGSStep does nothing for a whole block if there are

  zero diagonal entries in the diagonal block.

# Performance issues

## Performance issues

While a suboptimal implementation might be OK, if it is cleaner

and more readable these issues may decrease the performance

without any again.

* BlockGSStep and the derived classes loop over each row twice.

  Once for computing the residual, once for finding the diagonal entry.

# Design issues

## Design issues

Things that could be improved to make dune-solvers easier to use, understand,

and maintain.

* Implement (P)CG as IterationStep for the LoopSolver

* Make all public member variables protected and introduce public methods for access.

  version that is preconfigured with GS smoother and coarse grid solver.

* Remove MultigridStep::setNumberOfLevels(), this number is already

  fixed by the size of the transfer operator vector.

* MultigridStep hides member names from the base class by reusing

  them for different things

* MultigridStep works differently on the finest and the other

  levels. On the finest, the smoother is used with the rhs of

  the linear system and the current iterate as initial value

  to compute the new iterate. On the coarse levels it is used to

  solve the defect system approximately, i.e., it computes a

  correction with the residual and zero initial value.

  This should be unified or at least documented.

  Computing corrections also on the finest level might allow

  to solve systems really up to machine accuracy sice you

  have more digits for the correction.

# Naming of classes and members

## Naming of classes and members

* ignoreNodes_ should be called ignore_/ignoreIndices_/ignoreComponents_

# Missing features

## Missing features

A wish list for features that would be very welcome.

* Transfer operator assembler for adaptively refined grids (without refinementtype=copy)

  This is already implemented in dune-fufem. It could be moved here if the

  dependency on P1NodalBasis is removed.

* Nested iteration for ObstacleTNNMGStep.

* An interface to add custom output and termination criteria to the LoopSolver.