Multiple changes and advances

This collects the following changes:
 - Minor build system improvements, allowing us to revert FindEigen3.cmake to the original version supplied by Eigen.
 - New helpers: Selector, collapseJacs() and vertcat().
 - ImpesTPFAAD now uses new fluid interface (BlackoilPropsAd) and implements well equations.
 - Example program test_impestpfa_ad.cpp tests some of the new capabilities.
 - Remove some experimental dead ends.