Support for modulated IR sensor

1. Add code to support sensors that allow the IR output to be modulated, to
reduce the sensitivity to ambient IR. Use M558 P2 command to enable
sensor modulation. Also changed the sensor reading averaging code to
give more consistent z-height seeking.
2. Fix: a previous change to improve temperature reading accuracy resulted in the temperature readings no longer displaying as absolute zero if the thermistor was disconnected. This change restores that behaviour.

DO NOT PULL Release/RepRapFirmware-dc42.bin.