make nodal and qp clearer

[WilkAndy]

At the moment the Materials and Kernels use, e.g. "saturation" and "saturation_qp". The "_qp" is ok, but we should explicitly use the string "_nodal" for nodal quantities. It's clearer.

[cpgr]

I agree - I'll do this

[WilkAndy]

I've done a small bit. this is my convention:

_porosity_nodal(declareProperty("PorousFlow_porosity_nodal")),
_porosity_nodal_old(declarePropertyOld("PorousFlow_porosity_nodal")),
_dporosity_nodal_dvar(declarePropertystd::vector("dPorousFlow_porosity_nodal_dvar")),
_dporosity_nodal_dgradvar(declarePropertystd::vector("dPorousFlow_porosity_nodal_dgradvar")),
_porosity_qp(declareProperty("PorousFlow_porosity_qp")),
_porosity_qp_old(declarePropertyOld("PorousFlow_porosity_qp")),
_dporosity_qp_dvar(declarePropertystd::vector("dPorousFlow_porosity_qp_dvar")),
_dporosity_qp_dgradvar(declarePropertystd::vector("dPorousFlow_porosity_qp_dgradvar"))