Add safety guard against error-prone sensor data

[mwood77]

Cheap pressure sensors are prone to throwing trash data (they get shorted-out easily). We need to programmatically guard against this by setting duty cycle to 0% if this goes above a certain threshold.

Possible solution considerations:

• Verify the mV reading against group head temperature?
• Verify against total uptime & group head temperature
• check span between last (n)number of readings and current. If span is greater than (n), cut power?

[mwood77]

Current solution under test:

1. On boot, set constant INIT_TEMPERATURE as group head's current temp.
2. Compare previous pressure sensor input to current pressure sensor input (pressure in BAR)
3. If value difference is greater than 2.0, increment ERROR_COUNT (init @ 0).
4. If ERROR_COUNT is greater than or equal to 20 && current group head temperature is greater than INIT_TEMPERATURE + 10, set fail condition and shut off relay (turn off power).

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INIT_TEMPERATURE + 10 is needed, as known good pressure sensors throw a large amount of unreliable data during initial power up/heating; it takes some time for the sensor to stabilize during the first 5 minutes of operation.