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, 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.