don't use the 1kHz timer as it conflicts with other I2C devices

this allows for support of multiple sensors on a board

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

healthy flag made protected
healthy accessor fn added which also check latest calculated altitude
was ok

this is useful for pressure altitude calculations

used for auto baro calibration when disarmed

thanks to Mike McCauley for pointing this out

we switched to GPLv3 a long time ago, but neglected to update the
per-file license headers

thanks to Randy for spotting this

used for automatic barometric adjustment by a ground station equipped
with a barometer

this provides the scaling factor between equivalent and true airspeed
based on altitude

if not using an AVR CPU then use a more computationally expensive
altitude calculation, which is more precise at higher altitudes

- Define float versions of math functions to the double versions
  on AVR (eg. #define sinf sin).
- These macros appear to be missing in older versions of avr-libs.
- Include AP_Math.h rather than math.h to get these definitions.

- Allows use of hardware floating point on the Cortex-M4.
- Added "f" suffix to floating point literals.
- Call floating point versions of stdlib math functions.

this saves a few bytes of memory

this changes the barometer calculations to floating point. On a MS5611
this is actually about twice as fast as the previous 64 bit
calculations, but gains us more accuracy as we are able to take
advantage of sub-bit precision when we average over 8 samples.

we need to re-load the ground pressure and temperature from eeprom on
and airstart

this allows the barometer driver to calibrate and return altitude and
climb rate values. This will be used by the AHRS drift correction code
for vertical velocity

The climb rate uses a 5 point average filter