move namespace from "Dune" to "Opm"

Also, put the headers into the "opm/benchmarks" directory instead of
the previously used location (-> dune/opm_benchmarks)

this pull request finishes the transition to the "Opm" namespace, since the remaining modules used the "Opm" namespace from their inception (opm-polymer), are empty (opm-verteq, at least it's empty on github) are supposed to be using "Dune" (dune-cornerpoint) or use something different (eWoms, ResInsight).