Improve behavior with elevator angles beyond the stall angle

[chapulina]

See discussion in #63 (comment)

tl;dr: the elevator's angle range is +-15 deg, but we're observing behaviors beyond 0.17 rad / 9.7 deg (the stall angle) which we can't fully explain yet:

elevator = 0.26 rad (15 deg), prop omega = 30.0 rad/s

elevator = 0.15 rad (9 deg), prop omega = 30.0 rad/s