Depletion voltage determination from spline fit maximum curvature
This should be added as additional "fitfunction". The overloaded function in hex_values::fit_pad_value_curve() can take a function or a spline as argument, performs the required operations and gives back the resulting value. These functions have been created in commits 4e14a4da and 39280e6d. The spline of fit functions are modified in place and can then be plotted. The plotting in the depletion voltage option is at the moment only implemented for functions, so there we need some construction.