"unit: dBV" is very slow in spice_unit_unconvert

Original issue by jwise: https://github.com/efabless/cace/issues/53

---

For a simulation in which

```
variables {
name: freq
display: Frequency
unit: Hz

+
name: Vprobe
display: Vin
unit: dBV
[...]
}
```

... then it takes many tens of milliseconds per unit conversion to run spice_unit_unconvert on this value and plot a simulation output.

Locate the spice_unit_unconvert entry point and reproduce the dBV conversion described in this issue using the provided simulation variables. Measure the conversion time and compare it with the plotting path; done means the dBV conversion no longer takes many tens of milliseconds per value without changing the plotted output.

Written by the indexing model from the issue text.