If given build tree as Eigen3 root, find source

Eigen3 is a template-library, so we must compile the source code
directly together with ours instead of linking to a library.

If the build script gives us the location of a "built" Eigen3,
meaning a directory in which CMake has been run, locate the true
source directory from the cache entries.
Search after new Eigen3 versions in sibling dirs

If we have checked out a newer Eigen3 directory in a sibling directory
to ours, assume that this is because the system version is obsolete and
that we want to use this one instead.
Provide CMake find module for opm-autodiff

If the module root is not explicitly specified, the loading of its
configuration is delegated to a find module, which does a heuristic
search in known/surrounding directories. Most OPM modules use the
common logic in OpmPackage; this patch configures the search
specifically for opm-autodiff.
Merge pull request #444 from rolk/444_autodiff

Allow opm-autodiff to be used as a prerequisite