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.
Merge remote-tracking branch 'atgeirr/master'
Merge remote-tracking branch 'atgeirr/master'
White-space cleanup.

No functional changes.
Merge remote-tracking branch 'bska/master'
Merge pull request #1 from bska/master

Implement total Darcy flux calculation and minor build-system cleanup
Refine OpmFind according to Roland Kaufmann's suggestions.

This enables us to restore FindEigen3.cmake to its original
state (as copied from Eigen sources).