Lumping of richards mass, as per issue #2805 .  UserObjects are utilised rather than AuxVars which were my previous method.  This is much better from a user's perspective, since the UserObjects are the same as used by RichardsMaterial.  Also it gets around the strange problem of Saturation(Seff) not being defined at time=0, and the problem of >2 phases in OffDiagJacobian.  Also enhanced RichardsPorepressureNames to return MooseVariables.  Also added RichardsSatAux and RichardsSatPrimeAux, which are not actually needed for this UserObject-utilisation method, but may be useful for something else.  The mass lumping has been tested in the buckley-leverett tests (pgas is not written to exodus files due to it being basically undefined), and also the jacobians have been tested.  However, there is still work to do with updating latex doco, and i want to implement some more tests.  But this is probably a good time to push into devel.
Merge pull request #2810 from WilkAndy/feature_2805

Lumping of richards mass, as per issue #2805 .  UserObjects are utilised...
Merge commit 'a1215a952f3cd793acb1a2ed9b26f193c7a8d488'