dda_maths.h: Add comment on units of C0.
config.default.h: Add DEBUG_LED_PIN to the pinout section.
arduino_32U4.h: remove definition of DEBUG_LED_PIN from here, leaving it optional in config.h.
dda_lookahead.c: Eliminate debug crossF uninitialized 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 - M130 , I - M131, and D M132 parameters.
heater.*:Adjust DEFINE_HEATER macro to provide PID values.
heater.c: Add units comments and sum the PID in consistent units, changing internal kP,kI,kD meanings.
config.teensypp.h: Add note on compiling from the command line
heater.c: Parse PID table -Ki as a raw value.
heater.c, heater.h, gcode_process.c: Replace hard-coded PID I_LIM with adaptive anti-windup logic.
heater.c: Add some PID units notes.
heater.*: Remove PID I-limit traces.
heater.c:initialize heater_i to 0.
heater.c: Apply *16 factor to integral accumulator backcalculation (1Cs=16qCqs).
config.teensypp_drf.issue74.h: align with config for my machine.
Move the PID_SCALE_[PID] into heater.h from config.*, and apply them.
heater.*: change and doucment units for PID parameters
heater.c:Add #define BUMPLESS code for smooth transitions during adjustment of PID parameters.
heater.c:Adjust PID_SCALE_D to divide by TH_COUNT samples.
heater.*, config.teensypp.h: Switch from Ti,Td to kI, Kd.