[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
[rotorcraft] guidance_v: set some defaults that nobody changes anyway