Merge pull request #14 from atgeirr/master

Change use of Boost.Filesystem
change namespace from Dune to Opm
fix some fallout of the Dune->Opm conversion

the class->struct change avoids a CLang warning, the second change is
a class that is not tested within opm-porsol but required for opm-upscaling.
Merge pull request #18 from andlaus/fix_fallout

fix some fallout of the Dune->Opm conversion