heater.[ch]: Add units comments, and #define PID_SCALE_[PID] factors.
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.issue74.h: Add 'MCU_TARGET=atmega644p  make -f Makefile-AVR' command line note.
heater.*, config.issue74.h: Switch from Ti,Td to kI, Kd.  Add notes, and resolve TH_COUNT scaling of kD.
heater.c: back the conversion factors out of the PID calc that are now in PID_SCALE_[PID].
gcode_process.c: Add doc to M130-134, and note that for M130-132, S is scsled by PID_SCALE.
sermesg.c: Add documentation tag for variable floating point.
Adjust fixed point precision of M13x S??? values to int16_t.7
heater.c: Don't store Watts and t_dead fields from DEFINE_HEATER.