adapt to the removal of the unit system of opm-core
the one which is in opm-parser is now a drop-in replacement.
see OPM/opm-parser#936 for details.
jenkins build this opm-parser=936 opm-core=1085 please
hm, to me the issue reported by Jenkins seems to be unrelated to this PR.
well, problem sits in opm-parser #936 - trying to install a nonexistent file.