temp.c: fix typo.
config.teensypp.h: Modify pins to be more like Teensylu and Printrboard.
heater.c: Add comments about units of PID variables and constants.
simulator: Turn off printf format warnings

PC-based target emits warnings about printf formats, but there's nothing
to be done about them, really.  Turn them off for polite builds.
dda.c: Update links to 'Generate stepper-motor speed profiles in real time' David Austin 2004 Embedded article.
heater.c: Update link to 'PID without a PhD' Tim Wescott 2000 Embedded article.
mendel.c: Enable DEBUG_LED_PIN output if defined.
config.teensypp.h: Update to match testbed hardware.
dda.c: endstop stop more reliably.
gcode_parse.h: raise S word size.

Some M-codes apparently need this. Perhaps it could be avoided,
but it costs just 8 bytes binary size and 2 bytes RAM, so not
really a reason to make a headache.
Add testcases for velocity attained

WARNING this script will overwrite your config.h

The velocity-test.sh script will compile the simulator with a set
of canned config parameters and for a range of different ACCELERATION
values.  Then it runs the simulator against a range of different
target speeds, with and without lookahead enabled.  Finally it plots
the results in gnuplot to the screen.

The actual speed is calculated from the trace file using an awk
script to calculate the differential of the position w.r.t time.
Time is on the X axis and is given in minutes.  Speed on the Y
axis is given in mm/min (same as feedrate).
testcases/velocity-test: Vary STEPS_PER_MM

Some values for STEPS_PER_MM can cause overflow errors in the step-time
calculations.  Vary this compile-time constant during the simulation
in velocity-test to help identify where the useful ranges are.