Promote 'test_impestpfa_ad' to an 'example'

This was the original intention, but the file was misplaced during
the module reorganisation.
Build 'test_impestpfa_ad' along with the 'examples'
Implement first cut at Darcy (total) flux calculation.

This is compile-tested only.
The 'pflux' is really the hydraulic head.

Rename variable accordingly.
Move rel-perm and viscosity calculations closer to use

This is an aesthetic change, only.