Soft PWM: dither low bits when SOFT_PWM_SCALE>0 to enhance resolution

Update description of SOFT_PWM_SCALE in Configuration.h
Fix uneven duty cycle increase between 0 and 1 level in soft PWM

Old behaviour: duty cycle increases by 2 ticks/cycle between 0 and 1
level, then by 1 tick/cycle each next level. Full cycle on at 127 level.

New behaviour: duty cycle increases by 1 tick/cycle between any two
adjacent levels. Full cycle on at 128 level.
Soft PWM: add option to decimate heater PWM power range

Add option to decimate PWM duty cycle range to obtain better resolution
for low-power heaters, which use only a fraction of full power.
Note that each increase of SOFT_PWM_DECIMATE_... by one should be
accompanied by the corresponding twofold increase of Ki, Kp, Kd, PID_MAX,
and BANG_MAX parameters.

- Add SOFT_PWM_DECIMATE_HEATER_0 control to Configuration.h.
- Extend internal representation of PWM level from 7 to 15 bits in ISR
in tempertute.cpp.
- Turn PWM dithering on for all values of SOFT_PWM_SCALE.