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...
remove unused files 'fmatrixev_ext.{cc,hh}' and 'upscaling.hh'
make all headers independently includable
fix fallout from dune's GenericReferenceElement -> ReferenceElement renaming
Merge pull request #75 from andlaus/fix_headers

make the headers independently includable