Update comments in a couple of places.
Merge pull request #231 from bangerth/doc-update

Update comments in a couple of places.
add a test for the iterated IMPES solver
Merge pull request #232 from jdannberg/iterated_IMPES_test

add a test for the iterated IMPES solver
Handle change of output_interval during restart.
Merge pull request #236 from gassmoeller/visualization_fix

Always create a visualization output after restart if interval is set to 0