Use explicit string constructor for keywords SWFN, SOF2, and TUNING.

Keywords are supposed to be strings anyway, and it's better to enforce
this restriction explicitly rather than to rely on implicit conversion
from character literals.

opm/core/eclipse/EclipseGridParser.cpp

@@ -98,9 +98,9 @@ namespace EclipseKeywords
 	  string("EQUIL"),    string("PVCDO"),    string("TSTEP"),
 	  // The following fields only have a dummy implementation
 	  // that allows us to ignore them.
-          "SWFN",
-          "SOF2",
-          "TUNING"
+          string("SWFN"),
+          string("SOF2"),
+          string("TUNING")
         };
 
     const int num_special_fields = sizeof(special_fields) / sizeof(special_fields[0]);