Allow to specify directory for all projects in a suite with one parameter

This was originally intended as a comment to Lars' excellent suggestion in OPM/opm-upscaling#44 on how the name could be parsed into a "suite" (opm-) and a "project" part (-upscaling) and then the appropriate variable (OPM_ROOT) deduced from that. That would have avoided keeping a list of projects that possibly would have to be maintained.

However, the comment grew so large I figured I could just type it up myself and submit so it could be more easily reviewed!

This implementation also handles some possible corner cases, such as someone specifying --with-dune=/usr and expecting it to pick up a package installation from there.