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.
A static variable in a member function is problematic because if this class would be instantiated multiple times (e.g. in a Linux system running two controllers derived from this class for test purposes), both classes set this field and these kind of bugs are extremely hard to narrow down. It would be preferable to have a _control_position_last_called; member variable of the class (please also initialise to zero in the constructor).