Square size override only when elected

[mprib]

This may involve some slightly onerous GUI work/rework. The charuco widget has started to have some quick fixes pile up in it that will make this more challenging (but also more necessary).

By default there should be no override. The square size should just be an estimate based on the rows/cols and the board dimensions. If the user elects an override, then apply it. So there should be a new data element in the charuco board: override_square_size to accompany square_edge.