Second dterm pt1 (#5458)

* Second PT1 on DTerm

This PR replaces the default biquad filter with a second PT1 set to
200Hz.

Basically allows the user to enable a second, set point configurable,
PT1 type first order low-pass filter on DTerm.

This is useful because most noise in most logs arises from D, not P.

The default is set to on, at twice the normal Dterm setpoint.  This
provides greater Dterm cut than a single PT1, and twice the steepness
of cut above the second setpoint.  Modelling shows significant
reductions in higher frequency Dterm noise with only minor additional
delay.

The improvement in noise performance will be less than for biquad, but
the delay is considerably less.

If with the default settings the overall noise improves a lot, it may
be possible bring D both filtering set points to higher numbers (e.g.
140/280), or alternatively remove other filters such as the notch
filters, while maintaining an adequate level of control over noise.

* Update names, old defaults, fix whitespace

Defaults restored to biquad with second PT1 off.  ‘lpf’ retained as
abbreviation for values, otherwise generally remove ‘Filter’ where
redundant, replace ‘FilterLpf’ with ‘Lowpass’, etc, thanks Fujin and
DieHertz

* Remove underscore in lowpass_2, add hz to setpoint for lowpass

Thanks DieHertz

* completed replacing lpf with lowpass, added _hz to all lowpass set points in profile

Thanks DieHertz

* fix whitespace

fixed whitespace in settings.c

* whitespace attempt #57

* change lpf to lowpass where appropriate elsewhere

Note did not change OSD abbreviations, they are still LPF, and did not
change gyro_lpf anywhere.

* second attempt at a simple PT1 implementation

Basically copied from the DtermNotch implementation

* Second PT1 on DTerm

This PR replaces the default biquad filter with a second PT1 set to
200Hz.

Basically allows the user to enable a second, set point configurable,
PT1 type first order low-pass filter on DTerm.

This is useful because most noise in most logs arises from D, not P.

The default is set to on, at twice the normal Dterm setpoint.  This
provides greater Dterm cut than a single PT1, and twice the steepness
of cut above the second setpoint.  Modelling shows significant
reductions in higher frequency Dterm noise with only minor additional
delay.

The improvement in noise performance will be less than for biquad, but
the delay is considerably less.

If with the default settings the overall noise improves a lot, it may
be possible bring D both filtering set points to higher numbers (e.g.
140/280), or alternatively remove other filters such as the notch
filters, while maintaining an adequate level of control over noise.

* Rebase

* Remove underscore in lowpass_2, add hz to setpoint for lowpass

Thanks DieHertz

* completed replacing lpf with lowpass, added _hz to all lowpass set points in profile

Thanks DieHertz

* fix whitespace

fixed whitespace in settings.c

* whitespace attempt #57

* change lpf to lowpass where appropriate elsewhere

Note did not change OSD abbreviations, they are still LPF, and did not
change gyro_lpf anywhere.

* second attempt at a simple PT1 implementation

Basically copied from the DtermNotch implementation

* Whitespace fix - thanks, Ledvinap

* Fix PG issue

by moving added dterm_lowpass2_hz to bottom of struct

* Got rid of redundant indirection

* Fixed indentantion shifts

src/main/flight/pid.h

@@ -73,8 +73,8 @@ typedef struct pid8_s {
 typedef struct pidProfile_s {
     pid8_t  pid[PID_ITEM_COUNT];
 
-    uint16_t yaw_lpf_hz;                    // Additional yaw filter when yaw axis too noisy
-    uint16_t dterm_lpf_hz;                  // Delta Filter in hz
+    uint16_t yaw_lowpass_hz;                    // Additional yaw filter when yaw axis too noisy
+    uint16_t dterm_lowpass_hz;                  // Delta Filter in hz
     uint16_t dterm_notch_hz;                // Biquad dterm notch hz
     uint16_t dterm_notch_cutoff;            // Biquad dterm notch low cutoff
     uint8_t dterm_filter_type;              // Filter selection for dterm
@@ -105,6 +105,7 @@ typedef struct pidProfile_s {
     pidCrashRecovery_e crash_recovery;      // off, on, on and beeps when it is in crash recovery mode
     uint16_t crash_limit_yaw;               // limits yaw errorRate, so crashes don't cause huge throttle increase
     uint16_t itermLimit;
+    uint16_t dterm_lowpass2_hz;                // Extra PT1 Filter on D in hz
 } pidProfile_t;
 
 #ifndef USE_OSD_SLAVE