* Add Arlet's 6502 as Coriolis test bench.
* Fix imported notebooks
  * portfolio.ipynb:
    * Fix PDK module location
    * Add more structure and explanation.
    * Add standard cell library viewer.
  * inverter.ipynb:
    * Fix PDK module location
    * Add more structure and explanation, including Chips4Makers EDA vision.
    * Add design for equal drive current of inverter pull-up and pull-down
      transistor.
    * Add verification simulation and plot.
    * Correct typo.
* build fixes:
  * Added jupyter executable in Makefile.conf.
  * Fix design/Makefile.
  * Go hierarchically in design directory for full build and clean.