AP_Baro: make get_temperature() a const function
AP_Baro: add set_external_temperature()

this allows the use of an external temperature sensor for calibration
purposes, such as the sensor built in to the digital airspeed sensor.

The main affect this has is on the EAS2TAS calculation

The get_calibration_temperature() is used to choose either an external
temperature or an internal one. If an internal one is used then it is
clamped at no higher than 25 degrees C, to prevent hot electronics
on startup affecting altitude scaling and EAS2TAS
Plane: provide a temperature to baro from the airspeed sensor
AP_TECS: take load factor into account in min airspeed

this will push up minimum airspeed when turning
AP_SpdHgtControl: added load_factor in update_pitch_throttle()
Plane: use load factor to limit nav_roll_cd

this calculates the aerodymanic load factor from smoothed airspeed and
uses it to limit roll to keep it below the level where the load factor
would take us past the aerodymanic limit of the airframe
Plane: honor TKOFF_THR_MAX in set_servos()

fixes issue #1555