add extract to fix_group

this will allow users to define groups that are by default created internally by fixes, etc.

src/fix_group.cpp

@@ -251,3 +251,13 @@ void FixGroup::set_group()
 
   if (varflag) memory->destroy(var);
 }
+
+/* ---------------------------------------------------------------------- */
+
+void *FixGroup::extract(const char *str, int &unused)
+{
+  if (strcmp(str,"property") == 0 && propflag) return (void *) idprop;
+  if (strcmp(str,"variable") == 0 && varflag) return (void *) idvar;
+  if (strcmp(str,"region") == 0 && regionflag) return (void *) idregion;
+  return NULL;
+}