GCS_MAVLink: add MAV_CMD_NAV_SPLINE_WAYPOINT cmd

This is the same as a NAV_WAYPOINT except that the vehicle flies a
spline curved path to the destination
GCS_MAVLink: generate after adding NAV_SPLINE cmd
Mission: add decoding of NAV_SPLINE command
Copter: add do_spline_wp and verify_spline_wp functions
AutoTest: update copter_spline mission
AP_Common: remove spline flag from Location
Mission: remove support for Location spline flag
Copter: remove spline handling from regular wp_nav