interpolator overhaul
- now computes tool position
- split some USB descriptor stuff to a separate file
- motor direction is handled in the hardware
- new notation for structures initialization
- created a giant global structure for all CNC state variables
- USB :
 - interrupt are used for state change events
 - control transferts allow for cnc state query
 - now sends program size along code, detect program end
- webapp:
 - now displays tool position
 - displays a spinner when busy
 - 3D view displays tool position
- simulator now handles initial position