add the FILEUNIT keyword

currently, this keyword is basically ignored: setting the unit system
of a file to something different than the deck unit system will cause
`Opm::checkDeck()` to produce a warning.

opm/parser/eclipse/Parser/ParseContext.hpp

@@ -217,6 +217,12 @@ namespace Opm {
         */
         const static std::string PARSE_LONG_KEYWORD;
 
+        /*
+          The unit system specified via the FILEUNIT keyword is different from the unit
+          system used by the deck.
+        */
+        const static std::string UNIT_SYSTEM_MISMATCH;
+
         /*
           Some property modfiers can be modified in the Schedule
           section; this effectively means that Eclipse supports time