Binding setting the value of the millimeter unit to the vgdml load method.

Setting the millimeter value is a property dependent of the geometry conversion method, so it is more appropriate to pass it as a parameter to vgdml::Frontend::Load and only record it statically in GeoManager. Other conversion methods to be done in the same way. This does not affect the current default behavior with respect to units.

We will need a tag to be able to use this feature in AdePT.