make physical_quantity not wrap its base type
Merge remote-tracking branch 'eli/unitizing1' into unitizing1
update physical_quantity<> uses for it not wrapping the num type
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)