More ecal layer stack updates (#1846)

* switch PCB layers to 8 from 12

* update layer stack estimates specifically for slice test

- switch to Ti/Al instead of Carbon fiber
- just use layers of Copper and FR4 instead of attempting to construct a
  PCB material

* thickness of PCB is 1.2mm not 1.66mm

* add Kapton and a bit more docs

* fix kapton thickness, add mounting layers between Ti and Si

* numbat convert oz of Cu to mm for us

* have full three bi layers in bilayer_spec

* update documentation to module level, remove extra steps

* replace pcb() with pcb_unpacked() implementation

* move calculation of copper-in-PCB thickness to python

Author: tomeichlersmith