angle: Initial support for angle sensor calibration

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

docs/API_Server.md

@@ -354,7 +354,8 @@ A request may look like:
 and might return:
 `{"id": 123,"result":{"header":["time","angle"]}}`
 and might later produce asynchronous messages such as:
-`{"params":{"errors":0,"data":[[1290.951905,-5063],[1290.952321,-5065]]}}`
+`{"params":{"position_offset":3.151562,"errors":0,
+"data":[[1290.951905,-5063],[1290.952321,-5065]]}}`
 
 The "header" field in the initial query response is used to describe
 the fields found in later "data" responses.

docs/Config_Reference.md

@@ -4007,6 +4007,7 @@ Magnetic hall angle sensor support for reading stepper motor angle
 shaft measurements using a1333, as5047d, or tle5012b SPI chips.  The
 measurements are available via the [API Server](API_Server.md) and
 [motion analysis tool](Debugging.md#motion-analysis-and-data-logging).
+See the [G-Code reference](G-Codes.md#angle) for available commands.
 
 ```
 [angle my_angle_sensor]
@@ -4017,6 +4018,12 @@ sensor_type:
 #sample_period: 0.000400
 #   The query period (in seconds) to use during measurements. The
 #   default is 0.000400 (which is 2500 samples per second).
+#stepper:
+#   The name of the stepper that the angle sensor is attached to (eg,
+#   "stepper_x"). Setting this value enables an angle calibration
+#   tool. To use this feature, the Python "numpy" package must be
+#   installed. The default is to not enable angle calibration for the
+#   angle sensor.
 cs_pin:
 #   The SPI enable pin for the sensor. This parameter must be provided.
 #spi_speed:

docs/G-Codes.md

@@ -106,6 +106,27 @@ VAL=<value>`: Writes raw "value" into a register "register". Both
 "value" and "register" can be a decimal or a hexadecimal integer. Use
 with care, and refer to ADXL345 data sheet for the reference.
 
+### [angle]
+
+The following commands are available when an
+[angle config section](Config_Reference.md#angle) is enabled.
+
+#### ANGLE_CALIBRATE
+`ANGLE_CALIBRATE CHIP=<chip_name>`: Perform angle calibration on the
+given sensor (there must be an `[angle chip_name]` config section that
+has specified a `stepper` parameter). IMPORTANT - this tool will
+command the stepper motor to move without checking the normal
+kinematic boundary limits. Ideally the motor should be disconnected
+from any printer carriage before performing calibration. If the
+stepper can not be disconnected from the printer, make sure the
+carriage is near the center of its rail before starting calibration.
+(The stepper motor may move forwards or backwards two full rotations
+during this test.) After completing this test use the `SAVE_CONFIG`
+command to save the calibration data to the config file. In order to
+use this tool the Python "numpy" package must be installed (see the
+[measuring resonance document](Measuring_Resonances.md#software-installation)
+for more information).
+
 ### [bed_mesh]
 
 The following commands are available when the