rectpack for test structure assembly

[lukasc-ubc]

Implement something similar as in phidl, namely adding the ability to automatically compress layouts consisting of tons of test structures.

using: https://github.com/secnot/rectpack

similar to: https://github.com/amccaugh/phidl/blob/master/phidl/geometry.py

• input: a bunch of top cells
• extract floor plan, and assign "rid" to each cell / box
• compress