Revision 1701

comments for rev 1700.
Ran through spell checker for tutorial chapters.
Added in a tutorial chapter introducing the level set method with the Rayleigh-Taylor instability problem. Need to add python code at a later date for level set class.
Started to write up a chapter on model classes, e.g. StokesProblemCartesian, LevelSet, DurckerPrager, and TemperatureCartesian etc.