GCS_Mavlink: New message def: MAV_CMD_NAV_CONTINUE_AND_CHANGE_ALT
GCS_MAVLink: include units in CONTINUE_AND_CHANGE_ALT
GCS_MAVLink: code generation from XML for MAV_CMD_CONTINUE_AND_CHANGE_ALT.
AP_Mission: support for MAV_CMD_CONTINUE_AND_CHANGE_ALT
Plane: Implementation of MAV_CMD_CONTINUE_AND_CHANGE_ALT.
Plane: Keep throwing WP in front plane for CONTINUE_AND_CHANGE_ALT
Plane: fixed CONTINUE_AND_CHANGE_ALT command

current_loc should be used, not prev_WP_loc