Fix absolute moves

Absolute moves had two problems, described in #221 (closed) and #220 (closed) - briefly, an absolute move that didn't specify all 3 axes would fill in the blanks with zeros, which is not appropriate for an absolute move.

This MR fixes that by using None as default, and filling in the blanks only after converting absolute moves to relative ones - so axes that are not specified don't move, as expected. It also fixes the typing error that caused #221 (closed).

I have not used axes_to_array as it was clearer to integrate processing the arguments with filling in the default sensibly. However, I started by making axes_to_array handle None correctly, and added documentation - so I've left this change in.

Closes #220 (closed)

Closes #221 (closed)

Closes #222 (closed)