Copter: remove NAV_LAT and NAV_LON

Loiter and navigation controllers now combined
Copter: remove pid_nav_lat, pid_nav_lon
AC_WPNav: allow setting of horizontal velocity

Also added accessor function for waypoint radius parameter
Copter: remove WP_SPEED and WP_RADIUS parameters

AC_WPNav library has equivalent params with very similar names
AC_WPNav: use prev wp as origin for next wp
Copter: remove debug comment from RTL
AHRS: changed default RP and YAW gain to 0.3

this reduces the impact of hard acceleration on takeoff, and reduces
the impact of GPS lag

Note that this doesn't affect copters, as they override to 0.1
AHRS: removed AHRS_BARO_USE option

this option has caused users too much trouble. The vertical velocity
is too noisy from the baro
Copter: remove ahrs.set_barometer()
Plane: removed ahrs.set_barometer()
Plane: removed old HDNG2RLL PID control

not used with L1
autotest: precreate buildlogs directory
AP_L1_Control: change turn_distance() to be min of wp_radius and L1 distance

this gives less surprising behaviour for users
HAL_PX4: if we have lost RC input then set channel 3 to 900

this allows failsafe to kick in
Plane: allow for negative WP_LOITER_RAD for conter-clockwise loiter

this follows the same convention as missions
Plane : AP_L1_Control : Fix wrong way turn behaviour on loiter entry
L1_Control: cleanup some unused code and variables
scripts: added flashlog magnetic fit code

this runs the same mag offset code APM uses, but on a flashlog
AP_InertialNav: fixed build of example
AP_Math: handle NaN in constrain(), returning average

this makes it less likely a NaN will propogate