"unit: dBV" is very slow in spice_unit_unconvert

[jwise]

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.