fix(sbml-import): make TimeDerivative rhs have a per_time dimension

Otherwise we end up with dimensionally incorrect values:

```
<TimeDerivative variable="time" value="1"/>
```

It'll now be:

```
<TimeDerivative variable="time" value="tscale * 1"/>
```

where `tscale` has dimensions `per_time`.

src/main/java/org/neuroml/importer/sbml/SBMLImporter.java

@@ -831,7 +831,7 @@ else if (r.isAssignment()) {
             E.info(">>>> StateVariable "+sv);
             dyn.stateVariables.add(sv);
 
-            TimeDerivative td = new TimeDerivative(sv.getName(), "1");
+            TimeDerivative td = new TimeDerivative(sv.getName(), timeScale.getName() + "* 1");
             dyn.timeDerivatives.add(td);
         }