Support for modulated IR sensor

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.
Make disconnected thermistor show temp as absolute zero again

In a previous commit I changed the temperature calculation to give more
accurate readings at extreme values. This change meant that a
disconnected thermistor no longer shows as absolute zero. Coupled with
the change in thermistor beta value, this meant that a disconnected
extruder thermistor would show as -28.2C, which is above the usual error
threshold of -30C. This commit introduces a special case for a
disconnected thermistor and restores the reading to absolute zero for
that case.
New binary built from previous committed sources
Merge branch 'duet' of https://github.com/dc42/RepRapFirmware into duet
Merging David Crocker's latest changes and fixes.