Paper.js: Use compound path for drawing walls
Compute: use getSection
Fix interpolateLine when `self` is part of `line`
Python: Wrong error message
Move computeSections from tool to App
Add step-by-step debugging, preset points
Add an inner box

There are a couple of bugs
str(PolarLine): also toCartesian+universalCenter

We allow a universal center to be defined, so that we can output the
CartesianLine version
Fix Sections.intersects
Get section properly
Compute: have post-alter-sections function
Python: no need to print stack trace