cv_digitize: implement possibility to sample x axis when the unit is in `A` or `A / cm2`

[DunklesArchipel]

Currently, __main__.digitize_cv assumes that the traced plot is of type current (density) vs. voltage. In principle, it can also digitize voltage vs. current (density) plots. In that case, however, the sampling will fail, since the units on the x-axis are no longer in V.

We could implement the possibility to trace both options.

[DunklesArchipel]

I suggest that we add a UI module in svgdigitizer.general which allows digitizing any kind of voltage-current traces, where it does not matter if the voltage is located on the x-axis and a current on the y-axis.

The cv module in svgdigitizer.electrochemistry will inherit from UI and add a time axis based on a given scan rate.