AP_Baro: more precise altitude calculation on PX4

if not using an AVR CPU then use a more computationally expensive
altitude calculation, which is more precise at higher altitudes
AP_Baro: added units to baro documentation
AP_Baro: added get_EAS2TAS()

this provides the scaling factor between equivalent and true airspeed
based on altitude
AP_TECS: first implemention of TECS altitude control library
Plane: added new TECS altitude controller

Includes improvements to takeoff/launch detect logic to support hand
launching for X-8 flight
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
TECS-WIP : Fixed bug in initialisation of DT value
Plane: document ALT_CTRL_ALG=2
autotest: use TECS in autotest for plane