Option to reduce switching rate for heated bed if using relay

[bobc]

If using a relay for the heated bed, the switching time of the HB output is too fast. Due to noise reading the temp, it causes the relay to chatter around the threshold temperature. To reduce wear on the relay, the switching time should not be more than every second.

The thermal response time of the HB is quite slow, so does not need to be switch very fast.

This is not a problem if using a MOSFET, so I suggest having a config option to select tthe rate, e.g. heated_bed_rate = 1.0 // rate in Hertz.

A hysteresis range would also help.