Add rating headroom to parts

[ducky64]

Capacitors have an ad-hoc voltage rating derating.

Design questions:

• Where does this go, eg in application circuit or in the part itself? For capacitors - it goes in the passive-typed Capacitor object, which is low-level / common as it goes and provides a easy way to do a universal headroom setting via refinements
• What do the parameters mean in this context? In the capacitor sense, the voltage parameter would mean the voltage the device sees. Does this generalize to everything else, like inductors and FETs?