Primary increment is the right most bit, secondary is the next
rightmost. So you can increment 'A1' to 'A2' or 'B1' with
Shift-Alt-Scroll and Ctrl-Alt-Scroll respectively.
Remove the axis numbering code from ARRAY_OPTIONS
and place in a new class, ARRAY_AXIS. This keeps
the logic for the array item numbering separate from
the logic for the array item geometry.
This simplifies the logic in the ARRAY_OPTIONS class, which
no longer has to deal with the numbering of each axis.
