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.
Makefile-AVR: Update MCU_TARGET=at90usb1286 conditional.
simulator/heater_sim.c: Add notes that the heater isn't implemented in the simulator.
heater.*:Adjust DEFINE_HEATER macro to provide PID values.
config.issue74.h: M130-M136 set and report the 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, heater.h, gcode_process.c: Replace hard-coded PID I_LIM with adaptive anti-windup logic.
config.issue74.h: Add 'MCU_TARGET=atmega644p  make -f Makefile-AVR' command line note.
heater.c: Add some PID units notes.
heater.*: Remove PID I-limit traces.