More things to be done

[[Imported from SVN: r5917]]

TODO

-# 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.
@@ -9,7 +14,10 @@
 
 
 
-# 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.
@@ -22,7 +30,9 @@
 
 
 
-# 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.
@@ -38,19 +48,33 @@
   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.