AP_AHRS: added airspeed_sensor_enabled() function
AP_SpdHgtControl: added speed/height control interface

this will be used by TECS
Plane: changed to using a "aircraft parameters" structure

this structure can be passed to libraries to give them easy access to
critical user parameters
AP_TECS: updates to use new aircraft parameter structure
Plane: fixed TECS takeoff detection
AP_TECS: for _DT to at most 0.1