It didn't match the hangprinter-flex-compensation code
(which is the kinematics we use in Reprapfirmware).

The plotstuff.py script was used to make identical or near-identical
plots as are found in the hangprinter-flex-compensation repo.

We now also search for the max force within the algoritm.

... also tidies up flex_distance.py a little bit.

... since we're not completely sure about how well it works,
before we test it on data that was collected with known line tensions.

Used:
black -l 120 <file.py>

This reverts commit b7d57b2d.

Also makes spool_r limits depend on config variable

... all at z=0
This is easier to do by hand with measurement tape,
and it produces better accuracy results on my physical machine.