Import splines

this imports the cubic spline classes from eWoms. this PR is based on the property-system one but it in fact is rather independent: It only requires the opm/core/Exceptions.hpp header file which I did not want to add to two separate PRs. This PR also includes a fork of dune-common's unused.hh header file for pretty much the same reasons that the propertysystem PR contains a fork of the exception code. (IMHO we would need something like this anyway in the long run.)

this PR is a continuatuation of opm/opm-core#333 , which I accidentially closed in my stupidity...