AC_WPNav: Add EKF ground speed limit to loiter speed control

This is required to prevent the speed controller saturating the optical flow sensor during low altitude flying.
Copter: Add EKF ground speed limit to loiter speed control

This is required to prevent the speed controller saturating the optical flow sensor during low altitude flying.
AP_NavEKF: Expand EKF speed limit public method to handle control limits
AP_AHRS: Expand EKF speed limit public method to handle control limits
Copter: Expand EKF speed limit public method to handle control limits
AC_WPNav: Add nav velocity gain scaler to interfaces

Allows gains to be adjusted to compensate for optical flow noise
AC_AttitudeControl: Add EKF optical flow noise gain scaler

Allows gains to be adjusted to compensate for optical flow noise
Copter: Add velocity gain scaler to interfaces

Allows nav gain to be scaled to compensate for optical flow noise