[ins] alt_float: calc dt instead of using hardcoded value
[ins] calc dt instead of using INS_PROPAGATE_FREQUENCY
[ins] minor hff changes
[ahrs] pass dt to ahrs_propagate

not really properly taken into account for:
- int_cmpl_euler: this is very ugly and still has F_UPDATE 512Hz hardcoded, but nobody should use this one anyway..
- float_cmpl and int_cmpl_quat: still use AHRS_PROPAGATE_FREQUENCY to set the gains...
[rotorcraft] ahrs dt: only calc if USE_AUTO_AHRS_FREQ or !defined(AHRS_PROPAGATE_FREQUENCY)
[fixedwing] remove reduced propagation rate stuff

Also calculate dt if USE_AUTO_AHRS_FREQ or !defined(AHRS_PROPAGATE_FREQUENCY)
[test] update some mostly unused ahrs tests
[ahrs] also pass dt to update_accel and update_mag

Not so sure passing dt to the update functions is the best way to go...
Is only used to update the gains of the cmpl filters right now.
But it makes it possible to properly compute the gains and avoids having a dependency on sys_time in the AHRS.
[rotorcraft] only calc mag dt if actually using mag
[fixedwing] minor cleanup for ahrs_sim
[ins] alt_float, bound dt and used fixed GPS_DT if defined
[airborne] calc dt with usec resolution
Merge pull request #818 from paparazzi/propagate_dt

Pass dt to ahrs/ins propagation and updates instead of using the defines in the implementations.

The fixedwing ins_alt_float now uses the real dt for alt_kalman update from baro or gps.

For the AHRS: If the frequencies are not defines or USE_AUTO_AHRS_FREQ is TRUE, the dt is calculated from the system time.
Should be cleaner like this and less prone to misconfiguration, possible drawback however is that you get some more jitter on the DT.

The complementary filters now don't use the AHRS_x_FREQUENCY defines for calculation of the gains anymore and instead count the number of propagations since last update.