Adds support for multiple temperature sensors.

- The max number of temperature sensors supported is a configuration
  option in thermo_config.h

- Implements new smoothing which takes up less space and seems to
  work just as well.  Also works for multiple sensors.  However
  storage is statically defined, but configurable in thermo_config.h

- Adds halt_execution() to thermo_sensor.cpp.  This looks like a
  useful thing to have elsewhere in the project so I should stick this
  in a common source file for useful stuff