zero_current: Don't mark velocity as used

IS_SET marks the quantity as used, so it can't be modified
with e.g. boundary conditions. Using the `.isSet()` method
doesn't mark as used.

src/zero_current.cxx

@@ -56,7 +56,7 @@ void ZeroCurrent::transform(Options &state) {
 
   // Get the species density
   Options& species = state["species"][name];
-  if (IS_SET(species["velocity"])) {
+  if (species["velocity"].isSet()) {
     throw BoutException("Cannot use zero_current in species {} if velocity already set\n", name);
   }
   Field3D N = getNoBoundary<Field3D>(species["density"]);