verify_heater: Add initial support for verifying heaters and sensors

Add runtime checks to heaters and temperature sensors to check for
possible hardware faults.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

config/example-extras.cfg

@@ -122,6 +122,28 @@
 #   See the example.cfg for the definition of the above parameters.
 
 
+# Heater and temperature sensor verification. Heater verification is
+# automatically enabled for each heater that is configured on the
+# printer. Use verify_heater sections to change the default settings.
+#[verify_heater heater_config_name]
+#heating_gain: 2
+#   The minimum temperature (in Celsius) that the heater must increase
+#   by when approaching a new target temperature. The default is 2.
+#check_gain_time:
+#   The amount of time (in seconds) that the heating_gain must be met
+#   in before an error is raised. The default is 20 seconds for
+#   extruders and 60 seconds for heater_bed.
+#hysteresis: 4
+#   The difference between the target temperature and the current
+#   temperature for the heater to be considered within range of the
+#   target temperature. The default is 4.
+#check_time: 10
+#   The amount of time (in seconds) a heater that has reached the
+#   target temperature (as defined by the hysteresis field) may fall
+#   outside the target temperature range before an error is
+#   raised. The default is 10.
+
+
 # Multi-stepper axes. On a cartesian style printer, the stepper
 # controlling a given axis may have additional config blocks defining
 # steppers that should be stepped in concert with the primary