Makefile-AVR: Update MCU_TARGET=at90usb1286 conditional.
simulator/heater_sim.c: Add notes that the heater isn't implemented in the simulator.
sermesg.c: Add documentation tag for variable floating point.
gcode_process.c: Add doc to M130-134, and note that for M130-132, S is scsled by PID_SCALE.
gcode_parse.c: Debug S with serwrite_int32.
heater.[ch]: Add units comments, and #define PID_SCALE_[PID] factors.
heater.h: Cast the PID_SCALE_? types to int32_t
heater.c: Add units comments; Set PID defaults with SI units and PID_SCALE_[PID].
heater.*:Adjust DEFINE_HEATER macro to provide PID values.
heater.c:Use SI units for M130-M133
config.teensypp.h: Use heater config table with PID.