extruder: Initial support for "smoothed pressure advance"

Support averaging the extruder position over a time range to "smooth
out" the velocity changes that occur during pressure advance.

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

docs/Config_Changes.md

@@ -10,6 +10,9 @@ All dates in this document are approximate.
 chip id by default.  Update the "serial" setting in the "mcu" config
 section accordingly.
 
+20191121: The pressure_advance_lookahead_time parameter has been
+removed.  See example.cfg for alternate configuration settings.
+
 20191112: The tmc stepper driver virtual enable capability is now
 automatically enabled if the stepper does not have a dedicated stepper
 enable pin.  Remove references to tmcXXXX:virtual_enable from the

docs/G-Codes.md

@@ -156,9 +156,9 @@ The following standard commands are supported:
   Sets the target temperature for a heater. If a target temperature is
   not supplied, the target is 0.
 - `SET_PRESSURE_ADVANCE [EXTRUDER=<config_name>] [ADVANCE=<pressure_advance>]
-  [ADVANCE_LOOKAHEAD_TIME=<pressure_advance_lookahead_time>]`:
-  Set pressure advance parameters. If EXTRUDER is not specified, it
-  defaults to the active extruder.
+  [SMOOTH_TIME=<pressure_advance_smooth_time>]`: Set pressure advance
+  parameters. If EXTRUDER is not specified, it defaults to the active
+  extruder.
 - `STEPPER_BUZZ STEPPER=<config_name>`: Move the given stepper forward
   one mm and then backward one mm, repeated 10 times. This is a
   diagnostic tool to help verify stepper connectivity.

docs/Pressure_Advance.md

@@ -22,12 +22,11 @@ Use a slicer to generate g-code for the large hollow square found in
 speed (eg, 100mm/s), zero infill, and a coarse layer height (the layer
 height should be around 75% of the nozzle diameter).
 
-Prepare for the test by issuing the following G-Code commands:
+Prepare for the test by issuing the following G-Code command:
 ```
 SET_VELOCITY_LIMIT SQUARE_CORNER_VELOCITY=1 ACCEL=500
-SET_PRESSURE_ADVANCE ADVANCE_LOOKAHEAD_TIME=0
 ```
-These commands make the nozzle travel slower through corners and they
+This command makes the nozzle travel slower through corners to
 emphasize the effects of extruder pressure. Then for printers with a
 direct drive extruder run the command:
 ```
@@ -118,17 +117,6 @@ Important Notes
   enough torque to push the required filament. If this occurs, either
   use a lower acceleration value or disable pressure advance.
 
-* The pressure_advance_lookahead_time parameter controls how far in
-  advance to check if a head slow-down is immediately followed by a
-  speed-up - it reduces pointless pressure changes in the head. It is
-  recommended to follow the steps above so that it is set to zero
-  during tuning and to use the default (0.010) during normal prints.
-  It is possible to tune this setting - higher values will reduce the
-  amount of pressure change in the nozzle during cornering, but
-  setting it too high can cause blobbing during cornering. (Tuning
-  this value is unlikely to impact ooze.) The default of 10ms should
-  work well on most printers.
-
 * Once pressure advance is tuned in Klipper, it may still be useful to
   configure a small retract value in the slicer (eg, 0.75mm) and to
   utilize the slicer's "wipe on retract option" if available. These