Experimental filter-based rc channel smoothing

Adds an additional rc channel smoothing algorithm that can be used in place of the default rc interpolation. Utilizing a filter-based approach the smoothing has lower latency and is immune to loop time jitter that can introduce artifacts.  Additionally a smoothing filter is added to the setpoint derivative used to produce D-term setpoint weight resulting in a smoother effect on D.

The default setting is to use the previous interpolation logic and there are no changes unless the optional method is selected.

Configuration:

rc_smoothing_type: (INTERPOLATION | FILTER) - defaults to INTERPOLATION

rc_smoothing_input_hz: (0-255) - sets the rc channel input filter cutoff in Hz. Default value of 0 will enable auto calculation based on received RX frame rate.

rc_smoothing_derivative_hz: (0-255) - sets the setpoint weight derivative filter cutoff in Hz. Default value of 0 will enable auto calculation based on received RX frame rate.

rc_smoothing_debug_axis: (ROLL | PITCH | YAW | THROTTLE) - determines which axis is logged in the debug fields

Debug logging:

set debug_mode = RC_SMOOTHING

debug(0) = raw un-smoothed rc channel data
debug(1) = raw un-smoothed setpoint derivative
debug(2) = filtered setpoint derivative before applied to setpoint weight
debug(3) = auto-calculated filter cutoff frequency base after sampling the rx frame rate

Notes:

Currently only enabled for F4/F7 due to flash size limitations

Uses the rc_inter_ch parameter to determine which channels are smoothed (same as default interpolation logic)

The auto filter cutoff calculation will set a cutoff frequency of 30Hz for typical SBUS frames (9ms).  11ms Spektrum will calculate to approximately 25Hz. The user can manually enter the filter cutoffs to be used instead of the auto calculation.  The current default calculation was chosen as a good starting point but may be adjusted in the future.

Setting a lower cutoff frequency will result in more smoothing, but also more delay.

There currently isn't any support for receivers that change their rx frame rate dynamically.  So for CRSF users wishing to use this alternate smoothing method should change their settings to lock the rx frame rate for now. Support for auto-adjusting to new frame rates in flight will likely be added.

src/main/build/debug.c

@@ -71,4 +71,5 @@ const char * const debugModeNames[DEBUG_COUNT] = {
     "RTH",
     "ITERM_RELAX",
     "ACRO_TRAINER",
+    "RC_SMOOTHING",
 };