carousel.comp graycode zero

[andypugh]

Some tool changes use an all-zero Gracode to indicate the highest position. The compinent currently ignores a zero as invalid.
Add a flag to allow 0000 == num_pockets

Also, found while investigating, need to add code to explicitly set graycode mode rathe than it being the default. (only a problem with multiple toolchanges, the first of which is not graycode and a subsequent one is)

[andypugh]

Fixed by 2d6b598 (hopefully, I have no hardware to test)