Add a visitor class to replace units in statements

Some of the mod files (such as [this one](https://github.com/neuronsimulator/nrn/blob/master/share/examples/nrniv/nmodl/cabpump.mod)) have units directly in statements, for instance:

```plaintext
area1 = 2*PI*(diam/2) * 1(um)
```

We should add a visitor (that runs fairly early) which replaces these units with their actual numerical values since the syntax is not compatible with SymPy.