Initialise all heater model parameters in constructor so that on a Durt
0.8.5 heater 6 has sensible model parameters even though it is disabled
Fixed PWM frequenct on Duet 0.6/0.8.5
Fixed several bugs in file time stamping
Fixed M280 and M42 commands using unused heaster channels on Duet 0.6
and 0.8.5
Reduced the amount of I2C traffic, by using DueX5 interrupt line to tell
if new data is available and by only sending fan PWM values when they
change
Display TMC2660 driver status and DueX2/X5 stall status in M122 response
Removed E parameter from M574 command (use M558 I parameter instead)
Fixed response to M307 with H parameter only
