move registration of the EnableOpmRstFile parameter to EclBaseVanguard

before this patch, the parameter was registered by the problem but not
used there. Since this is quite confusing, let's move registration to
where the parameter is actually used, i.e., the vanguard.

ebos/eclbasevanguard.hh

@@ -104,6 +104,8 @@ public:
                              "The name of the file which contains the ECL deck to be simulated");
         EWOMS_REGISTER_PARAM(TypeTag, int, EclOutputInterval,
                              "The number of report steps that ought to be skipped between two writes of ECL results");
+        EWOMS_REGISTER_PARAM(TypeTag, bool, EnableOpmRstFile,
+                             "Include OPM-specific keywords in the ECL restart file to enable restart of OPM simulators from these files");
     }
 
     /*!