* Move coefficient computation into RuinaNonlinearity
* Allow some friction data to vary in space

x/|x| isn't that big a deal and the logic was a gigantic mess

As a consequence, the nonlinearity is only assembled once

Also kill the kink logic and thus minimisation2

Kill tests that use the old interface

Since 0 * nan = nan

x/|x| is only a problem if x == 0
just divide by |x|, no matter what non-zero value it is