Implemented altitude and velocity hold mode

Implemented Fixedwing Velocity and Altitude hold mode based on fw_pos_control_l1. 

In this mode throttle controlls airspeed between min and max speeds (set by FW_AIRSPD_MAX and FW_AIRSPD_MIN) and pitch controls vertical speed between FW_T_CLIMB_MAX and FW_T_SINK_MAX.
What happens if dt == 0.0f? Because a later adopter might use dt differently, (e.g. divide by it), a value of 0.00001f would be preferred. This is still an extremely small value, but doesn't lead to arithmetic corner cases.