Add another testcase, add lookahead-warmup.

Lookahead-warmup = two lines of G-code which take just long enough
to allow the intended movements to be added with look-ahead.
Add a draft of a script running testcases in SimulAVR.

This script also asks SimulAVR to generate pin traces of the X
and Y axis and even processes this data into a data file with
time, position and current speed of each axis.

Missing:

- Acceleration calculation.

- LOTS of polish.
run-in-simulavr.sh: also rewrite VCD file to add speed values.

This is quite handy, as you can see accurate speed ramps in
GTKWave now.
run-in-simulator.sh: use stdin/stdout for the data file, too.
run-in-simulator.sh: run (only) selected files.
run-in-simulator.sh: express velocities in mm/min, too.

Uh, this bloats the resulting VCD file by 50% ... ;-)
Turn dead-code removal back on when not simulavr
datalog: add a tip showing gnuplot usage

Add a comment to the datalog output showing how it can be viewed with
gnuplot.  It would be trivial to add this as a .plot script, but it's
even easier in the datalog output.
simulator: Make time-scale=0 a cmdline option

Teach the simulator to run with no delays when time-scale=0.

Let the user specify the time-scale on the command line.
gcode_parse.c: assume G1 only for stuff which might actually move.

Previously, even comments were run through dda_create(), quite a waste
of time.
dda_create(): store prev_dda locally.

This obviously requires less place on the stack and accordingly a
few CPU cycles less, but more importantly, it lets decide
dda_start() whether a previous movement is to be taken into account
or not.

To make this decision more reliable, add a flag for movements done.
Else it could happen we'd try to join with a movement done long
before.
dda_queue.c: run every queued thing through dda_create().

This allows dda_create() to track things queued but not being a movement.
Important for lookahead.
Makefile: get rid of the config.h check.

This doesn't work well for developers, because config.h.default is
rewritten all the time when switching branches.