Baro event handling using ABI interface

Main benefits:
* less crappy handler functions in main.c files
* possibility to use multiple sources of sensors (integrated to the board or not)
* the filters don't need to know who is sending the raw values (not completely true with the old alt_float filter yet)

+ the pressure are now standardized in Pascal and the standard atmosphere model is used
Some stuff to do before using: find the default conversion factor to convert ADC values to Pascal (mostly older boards/baro, recent digital sensors can output pressure in Pascal already)