WIP: Allow user to export individual CV and IV plots (but the same ones that would be plotted by --detailed)

I think there is already functionality for this but it seems to be implemented in a different part of the code base so that there's no good way to get a single pdf file with a depletion voltage fit in it.

My first commit just writes out such a file for every pad, a better solution could be to write out a file for each special pad. Or even better we unify the different plotting functions so that a CV plot is always generated by the same function.