consolidate the unit system to opm-parser

since the unit code within opm-parser is now a drop-in replacement,
this simplifies things and make them less error-prone.

unfortunately, this requires quite a few PRs. (most are pretty
trivial, though.)

opm/core/wells/WellsManager_impl.hpp

@@ -1,4 +1,4 @@
-#include <opm/core/utility/Units.hpp>
+#include <opm/parser/eclipse/Units/Units.hpp>
 #include <opm/core/grid/GridHelpers.hpp>
 
 #include <opm/common/ErrorMacros.hpp>