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.
This field should be initialised in the constructor (e.g. to zero). Without initialisation it could be any number, and its better to have a calculation with a wrong holding altitude than with NaN, as the NaN value could propagate in time, while the initial wrong altitude will only lead to a short spike in corner cases.