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 gnuplot script to explore approximate_distance.

Error is less than 5% for explored range [0,200000].
approx_distance: much more research into dist approximation

approximate_distance.plot now derives the appropriate values to
be used for 8-way fine-tuning of the fast distance approximation
algorithm.  It produces 3 graphs:

  * A unit circle chart showing the locations of the 8
    sample points chosen for the algorithm.

  * A 2D graph of the measured actual error values using the
    derived values and the approximation algorithm with X
    fixed at 10000 and Y along one axis.  Note the scale
    of this graph is in percent.  The y-axis ranges from 0 to
    0.4%.  The maximum error is less than 0.15%.  That is,
    when the actual distance is 100000, the approximated
    value will be between 99850 and 100150.

  * A 3D graph of measured actual error values for all X and Y
    between 0 and 2000.  One of these graphs is zoomed in to show
    the noise of the low level, and another one is zoomed out
    to show just how mirror-smooth it really is.
approx_distance: Improve accuracy of 2d and 3d funcs

Fine-tuning the approx-distance 2d function gives accuracy better
than 99.7% in all cases.  Extending this to the 3d version gives
it also accuracy better than 99% in all cases.  This is an
improvement on the old code which achieved accuracies better
than 95% in most cases.