SRV_Channel: added SERVO_DEF_RATE parameter

this allows for the default rate of servo outputs to be set. This is
needed for some planes where 50Hz isn't enough (eg. tailsitters)

libraries/SRV_Channel/SRV_Channel.h

@@ -391,6 +391,7 @@ private:
     
     // this static arrangement is to avoid having static objects in AP_Param tables
     static SRV_Channel *channels;
+    static SRV_Channels *instance;
     SRV_Channel obj_channels[NUM_SERVO_CHANNELS];
 
     static struct srv_function {
@@ -402,6 +403,7 @@ private:
     } functions[SRV_Channel::k_nr_aux_servo_functions];
 
     AP_Int8 auto_trim;
+    AP_Int16 default_rate;
 
     // return true if passthrough is disabled
     static bool passthrough_disabled(void) {

libraries/SRV_Channel/SRV_Channel_aux.cpp

@@ -151,6 +151,8 @@ void SRV_Channels::update_aux_servo_function(void)
 /// Should be called after the the servo functions have been initialized
 void SRV_Channels::enable_aux_servos()
 {
+    hal.rcout->set_default_rate(uint16_t(instance->default_rate.get()));
+    
     update_aux_servo_function();
 
     // enable all channels that are set to a valid function. This

libraries/SRV_Channel/SRV_Channels.cpp

@@ -25,6 +25,7 @@
 extern const AP_HAL::HAL& hal;
 
 SRV_Channel *SRV_Channels::channels;
+SRV_Channels *SRV_Channels::instance;
 bool SRV_Channels::disabled_passthrough;
 bool SRV_Channels::initialised;
 Bitmask SRV_Channels::function_mask{SRV_Channel::k_nr_aux_servo_functions};
@@ -102,6 +103,14 @@ const AP_Param::GroupInfo SRV_Channels::var_info[] = {
     // @User: Advanced
     AP_GROUPINFO_FRAME("_AUTO_TRIM",  17, SRV_Channels, auto_trim, 0, AP_PARAM_FRAME_PLANE),
 
+    // @Param: _DEF_RATE
+    // @DisplayName: Default output rate
+    // @Description: This sets the default output rate in Hz for all outputs. 
+    // @Range: 25 400
+    // @User: Advanced
+    // @Units: Hz
+    AP_GROUPINFO("_DEF_RATE",  18, SRV_Channels, default_rate, 50),
+    
     AP_GROUPEND
 };
 
@@ -110,6 +119,7 @@ const AP_Param::GroupInfo SRV_Channels::var_info[] = {
  */
 SRV_Channels::SRV_Channels(void)
 {
+    instance = this;
     channels = obj_channels;
     
     // set defaults from the parameter table