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.
dda: Reduce code duplication and simplify

There are three locations in the code that repeat a pattern of
"If z=0 then use 2d-approx(dx,dy), else if x==0 && y==0 then use dz,
else use 3d-approx".

Teach approx_distance_3 to detect these conditions for us and apply
the same logic.  Replace the three call locations with a simple call
to approx_distance_3.

Binary size for the LOOKAHEAD case drops by almost 400 bytes:

  old:  FLASH : 21242 bytes          149%       70%        34%       17%
  new:  FLASH : 20844 bytes          146%       68%        33%       17%

The size for non-LOOKAHEAD drops by 40 bytes:

  old:  FLASH : 16592 bytes          116%       55%        27%       13%
  new:  FLASH : 16552 bytes          116%       54%        27%       13%

We can actually do a little better if we consider the zero-ness of all
three axes, but this does make the code a little bit bigger.  Another
change will consider that option. This change simply tries to mimic
the existing functionality.
dda: Generalize the zero-distance case

When approximating distances, if any axis delta is zero we can
choose to consider only the remaining one(s).

This does increase the code size a few bytes, but is arguably more
accurate in specific "lateral move" cases.
Add gnuplot script to explore approximate_distance

Error is less than 5% for explored range [0,200000]
approx_distance_3: Accept signed and convert to unsigned

Simplify further (and save more bytes) by teaching approx_dist_3 to
accept signed values and normalize them for us.  Maybe we should
also teach approx_dist (2d) to do the same, but I did not do that here.