Fix an issue with calculate_duration_for_displacement and calculate_v…

…elocity_end_for_displacement using displacement instead of duration when trying to determine whether max-speed is reached before the destination is reached (displacement is ambiguous and can represent movement in either direction).

Various small code cleanups.

Fix a bug with trying to add jumps to non-floor start/end surfaces; F…

…ix a surface-state bug with inconsistent touch and grab state.

Update README

Update README