This address issue #2753.  Time constants now can be modified using Vari...

...able values.  Also i moved all but one of the desorption tests from the combined module into the checmical_reactions module.  The remaining one i renamed chemical_reactions_richards because it depends on desorption and the richards equation.  Also added doxygen comments into the desorption .h files, and removed some whitespace stuff from the .C files