Update TODO list

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

  This could be done in one loop.

* The ProjectedBlockGSStep 'solves' the local obstacle problems

  by a local projected GS with a fixed number of 20 iteration steps.

  For scalar valued problems one would be enough. Even for vector valued

  problems one might still be OK but much cheaper. (Does this have any

  performance impact on linear elasticity problems?)

  by a local projected GS with a fixed number of 20 iteration steps

  for blocksize!=1. Even in this case using only one step might still

  be OK but much cheaper. (Does this have any performance impact on

  linear elasticity problems?)

* 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.