Fixes #3467

moved tensor_mechanics_j2plasticity test from combined module into tensor_mechanics module

added derivatives of secondInvariant into RankTwoTensor

added doco and refactoring of FiniteStrainPlasticMaterial.
Note that there is a sign difference in the definition of "fq" compared with previous versions.  @cpritam and @WilkAndy believe this current version is correct.

this made necessary some tweaking in FiniteStrainRatePlasticMaterial

also changed one doxygen line in RankTwoTensor.h