Electrical units for part values

I made a tiny change in SKiDL: part values (like resistance, capacitance and inductance) can now be numerical values. They are transformed into strings when a netlist is generated. So anything that has a __str__ method will work as a part value. That means values can have units now, like the ones supported by PySpice and Pint.

This small change makes it a little less cumbersome to calculate part values when you’re doing generative circuits. And Pint supports measurements which are made up of a mean value and a variance, which adds an interesting capability if you want to see how performance is affected by part tolerances.

This change is currently in the development branch:

pip install git+https://github.com/xesscorp/skidl@development