Remove mm_per_unit (define mm or in instead)

[kr15h]

Hi, how about removing the mm_per_unit in examples and have a unit setter, such as 'unit = "mm"'; I would love the possibility to be able to work in millimeters.

[kr15h]

It also does not look like 'mm_per_unit' is used anywhere. The 25.4 constant is hardcoded always and the value in global_state never gets used anyways.