/up and /ceil now use less glass

no glass is used if the player is allowed to fly, which is usually the case in creative mode.
In that case, the player is simply set to flymode, no glass needed.
The old behaviour can be restored by passing the -g (for glass) flag.
API behaviour is unchanged, but there is a new overload with a boolean "alwaysGlass" flag now.

If anyone can think of a better name for the floatAt method or the -g flag, that'd be much appreciated, since I'm not really happy with those