RichardsPorepressureNames changed to RichardsVarNames, and various internal methods renamed.  This meant all input files had to be changed, and various pieces of the code too.  So this is a large rather boring commit
RichardsMaterial defines a mass variable and derivatives of it which are used in RichardsMassChange
RichardsMaterial defines a flux variable and lots of derivatives of it which are used in RichardsFlux
RichardsMaterial defines pressure, saturation, effective saturation, relative permeability and density, and derivatives of those, that are used in RichardsBorehole, RichardsPiecewiseLinearSink, RichardsPolyLineSink, etc
No doubt there will be more changes to be made, but for the present time this fixes #3228
Merge pull request #3235 from WilkAndy/devel

Fixes #3228 .  This is a mammoth refactor!
Merge commit 'e8bceee83470894b24c8ff3084bf708027b34f3d'