require opm-core from the opm-material module

this is required to use the exception code of opm-core in
opm-material. Also, the Dune prerequisites of opm-material can be
removed once PR #345 is merged into opm-core...
Merge pull request #363 from andlaus/require_opm_core_in_opm_material

require opm-core from the opm-material module