[ahrs] float_dcm: fix comment
[dox] imu units
[dox] fix some units
[conf] Quad_LisaMX with F4 and mlkf
[nps] calc magnetic field at location
[nps] get rid of stupid warnings
[nps] add math/pprz_geodetic_wmm2010.c
[build] don't complain on build if dfu-util is not found
[nps] set gps week and tow

currently still hardcoded to May 2014, needs to be implemented properly
[messages] GPS_INT: pacc, sacc are unsigned

thanks Antoine
[ahrs] do not require AHRS_H_x if mag is not even used
[conf] update some example airframes
[spektrum] fix some pin usage
[ins] fix low_pass_filter dox and configurable hff cutoff freq

closes #738
[filters] explicitly use float constants
[gps] GpsIsLost as static inline
[actuators] motor_mixing: trim defaults to zero
[airframes] remove redundant example
[rotorcraft] error on negative gains
[conf] enable mag for quadshot and add it to tests/examples