fixed frame length to be 1/30 sec not 1/30 time-unit
unitified tile_physics; it compiles

I don't know whether it works properly because I haven't made everything
   else compile yet
This unitification was kludgy because the tile physics still refers to
   frame lengths, and I may have changed some constants to be more
   physically accurate (which might change behavior or even break things)
deleted unused stuff
finished unitizing; it compiles! However...

I've made a type named "velocity" that has a name conflict with other
   things, and used a hack to work around that.
   Isaac and I should discuss what names to use.

No major behavior problems detected upon running it, although I may have
   changed some physical constants (especially if they were incorrect
   before this.)
renamed things to avoid name conflicts (velocity -> velocity1d, ...

velocity -> velocity1d, acceleration -> acceleration1d
Merge remote-tracking branch 'eli/unitizing1' into unitizing1

Conflicts:
	object_motion.cpp
	world_constants.hpp