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to ours, assume that this is because the system version is obsolete and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
Only add linker option if it is not really there

This code is run unconditionally each time we do a reconfigure; if the
option is added at each time, they will accumulate (needlessly) on the
command-line, making it harder to inspect the log.
Merge pull request #445 from rolk/445_asneeded

Only add linker option if it is not really there
Property system: some simplifications and bugfixes

this simplifies the template meta programming a bit. (Which does not
mean that it suddenly gets easy.) The main motivation for this work is
to make the splices feature work properly which allows eWoms to
properly select the spatial discretization at any point.

Further, the number of type tags a node can inherit from now is truely
unlimited thanks the the generic tuple reversal class provided by
https://sydius.me/2011/07/reverse-tuple-in-c/ . (before, the maximum
number of nodes from which a type tag could inherit was 11, so this
limitation did not really matter in practice.)
Merge pull request #446 from andlaus/propertysystem_fixes

Property system: some simplifications and bugfixes
added: nonstandard 'RHO' field in eclipse grid parser

note that as this is a nonstandard field, there is no unit conversion
handling. use with caution
Merge pull request #447 from akva2/add_rho

added: nonstandard 'RHO' field in eclipse grid parser