heater_sim.c: Note that the heater isn't implemented in the simulator.
dda_maths.h: Add comment on units of C0.
dda_lookahead.c: Eliminate debug crossF variable compile warning.

Fix:
  dda_lookahead.c:327:17: warning: 'crossF' may be used
  uninitialized in this function [-Wmaybe-uninitialized]
       sersendf_P(PSTR("Initial crossing speed: %lu\n"), crossF);
                 ^
gcode_process.c: Add comment on units of P, I, and D parameters.
config.default.h: Add DEBUG_LED_PIN to the pinout section.
config.*.h: extend DEBUG_LED_PIN comment to all config templates.
dda: Move axis calculations into loops.

Clean up code to reduce duplication by consolidating code into
loops for per-axis actions.
home: Reduce duplication with axis arrays
datalog: show traced data as signed ints, not unsigned
Rename confusing variable name

'all_time' sounds like forever to me, but this variable really
tracks the last time we hit one of "all the axes".  It sticks
out more now in looping, so rename it to make sense.
Trivial cleanups

Move some code around, fix some formatting, and hide a couple
of variables when they're not being used.