Migrating to coupledNodalValue interface

RichardsVarNames now uses coupledNodalValue.
RichardsPiecewiseLinearSink, RichardsBorehole, RichardsFullyUpwindFlux,
RichardsLumpedMassChange altered to use this.

Note that RichardsMaterial does not use this, but still calculates
things at quadpoints, even if only nodal quantities are required by BCs,
Kernels, etc.  I need to refactor this stuff and include a new
nodal-only Material, I think.

Closes #4038
Formatting according to code standards