fan: Allow the fan shutdown_speed to be configured

Add a shutdown_speed config option to fans so that users can specify
the speed on a shutdown event.

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

config/example-extras.cfg

@@ -193,12 +193,17 @@
 
 # Heater cooling fans (one may define any number of sections with a
 # "heater_fan" prefix). A "heater fan" is a fan that will be enabled
-# whenever its associated heater is active. In the event of an MCU
-# software error the heater_fan will be set to its max_power.
+# whenever its associated heater is active. By default, a heater_fan
+# has a shutdown_speed equal to max_power.
 #[heater_fan my_nozzle_fan]
-# See the "fan" section for fan configuration parameters.
-#pin: ar4
-# The remaining variables are specific to heater_fan.
+#pin:
+#max_power:
+#shutdown_speed:
+#cycle_time:
+#hardware_pwm:
+#kick_start_time:
+#   See the "fan" section in example.cfg for a description of the
+#   above parameters.
 #heater: extruder
 #   Name of the config section defining the heater that this fan is
 #   associated with.  The default is "extruder".
@@ -212,18 +217,23 @@
 
 
 # Temperature-triggered cooling fans (one may define any number of
-# sections with a "temperature_fan" prefix). A "temperature fan" is
-# a fan that will be enabled whenever its associated sensor is above
-# a set temperature. In the event of an MCU software error the
-# temperature_fan will be set to its max_power.
+# sections with a "temperature_fan" prefix). A "temperature fan" is a
+# fan that will be enabled whenever its associated sensor is above a
+# set temperature. By default, a heater_fan has a shutdown_speed equal
+# to max_power.
 #[temperature_fan my_temp_fan]
-# See the "fan" section for fan configuration parameters.
-#pin: ar4
-# See the "heater" section for details about the sensor_type and
-# sensor_pin parameters.
+#pin:
+#max_power:
+#shutdown_speed:
+#cycle_time:
+#hardware_pwm:
+#kick_start_time:
+#   See the "fan" section in example.cfg for a description of the
+#   above parameters.
 #sensor_type: EPCOS 100K B57560G104F
 #sensor_pin: analog13
-# The remaining variables are specific to temperature_fan.
+#   See the "heater" section for details about the sensor_type and
+#   sensor_pin parameters.
 #min_temp: 0
 #max_temp: 100
 #   The maximum range of valid temperatures (in Celsius) that the
@@ -234,12 +244,13 @@
 #   that reasonable temperatures do not result in an error. These
 #   parameters must be provided.
 #target_temp: 40.0
-#   A temperature (in Celsius) that will be the target temperature
+#   A temperature (in Celsius) that will be the target temperature.
+#   The default is 40 degrees.
 #max_speed: 1.0
 #   The fan speed (expressed as a value from 0.0 to 1.0) that the fan
 #   will be set to when the sensor temperature exceeds the set value.
 #   The default is 1.0.
-#min_speed: 1.0
+#min_speed: 0.3
 #   The minumin fan speed (expressed as a value from 0.0 to 1.0) that the fan
 #   will be set to when the sensor temperature is the set value.
 #   The default is 0.3.
@@ -249,10 +260,10 @@
 #pid_Kp: 40
 #   Kp is the "proportional" constant for the pid. This parameter must
 #   be provided for PID heaters.
-pid_Ki: 0.2
+#pid_Ki: 0.2
 #   Ki is the "integral" constant for the pid. This parameter must be
 #   provided for PID heaters.
-pid_Kd: 0.1
+#pid_Kd: 0.1
 #   Kd is the "derivative" constant for the pid. This parameter must
 #   be provided for PID heaters.
 #pid_deriv_time: 2.0
@@ -263,6 +274,7 @@ pid_Kd: 0.1
 #   The maximum "windup" the integral term may accumulate. The default
 #   is to use the same value as max_power.
 
+
 # Additional micro-controllers (one may define any number of sections
 # with an "mcu" prefix). Additional micro-controllers introduce
 # additional pins that may be configured as heaters, steppers, fans,