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 approach is problematic because all the fields need to be set correctly in multiple places. How about doing this with an ENUM variable as class member instead, which encoded the current control mode? The flag could be set in the main loop independent of the control mode at the end of the while() loop, and each control mode could just query if the current control mode == last control mode.

This would be much less error prone and less code and ensure that if someone adds new functionality, the mode detection logic is not broken in the future.