make the accZ lpf used for althold configurable

set the variable accz_lpf_cutoff to the desired cutoff frequency
this can help to make althold smoother on copters with lots of
vibrations
althold constants updated

baro velocity now limited to 15m/s
d-term is now smaller
slow change althold now controls velocity directly

The throttle stick input now directly sets the vertical velocity
setpoint of the copter. (alt_hold_fast_change = 0)
Merge pull request #150 from luggi/better_althold

Better althold