FlightTaskManual: Yaw lock with deadzone and expo

More than a year ago I started the easy to use math::Functions to handle
the always used mathematical SISO functions to be tested and available.

I switched x, y and z stick input to the freesh programmed deadzone and
exponential functions from the library to unify and clarify their use.
I just realized yaw was left over because it lead to a drift problem in
certain new use cases.

Now I'm just adding the yaw stick to the already well working method.

src/modules/mc_pos_control/mc_pos_control_params.c

@@ -550,6 +550,22 @@ PARAM_DEFINE_FLOAT(MPC_XY_MAN_EXPO, 0.0f);
  */
 PARAM_DEFINE_FLOAT(MPC_Z_MAN_EXPO, 0.0f);
 
+/**
+ * Manual control stick yaw rotation exponential curve
+ *
+ * The higher the value the less sensitivity the stick has around zero
+ * while still reaching the maximum value with full stick deflection.
+ *
+ * 0 Purely linear input curve (default)
+ * 1 Purely cubic input curve
+ *
+ * @min 0
+ * @max 1
+ * @decimal 2
+ * @group Multicopter Position Control
+ */
+PARAM_DEFINE_FLOAT(MPC_YAW_EXPO, 0.0f);
+
 /**
  * Altitude for 1. step of slow landing (descend)
  *