Enable PID debugging for heated bed

Marlin/Configuration.h

 // so you shouldn't use it unless you are OK with PWM on your bed.  (see the comment on enabling PIDTEMPBED)
 #define MAX_BED_POWER 255 // limits duty cycle to bed; 255=full current
 
+//#define PID_BED_DEBUG // Sends debug data to the serial port.
+
 #ifdef PIDTEMPBED
 //120v 250W silicone heater into 4mm borosilicate (MendelMax 1.5+)
 //from FOPDT model - kp=.39 Tp=405 Tdead=66, Tc set to 79.2, aggressive factor of .15 (vs .1, 1, 10)