look-ahead: change crossing speed calculation algorithm.

The new one solely looks at speed differences of individual axes.
This means individual jerks for each axis (good!) and relative
simple maths (also good!).

For details and maths, see comments in the code and
https://github.com/Traumflug/Teacup_Firmware/issues/45 .
look-ahead: move jerk definitions into config.h templates.