#56 Consistent units

[Steve McLaughlin]

The units ComboBox on the GUI only affects the design mode units but it should also apply to GCode and 3D translation.

Also consider specifying units in shapebot.properties.

[Håvard N. Jakobsen]

I'm not sure that the GUI unit should affect the g-code unit as they may not be the same ie. you would normally work in inches as most shapers do, however the aku, aps3000 and dsd shaping machine all work in mm units. However it might be useful to set the unit that we use when we export to g-code somewhere.