Added stage export option to disable authoring of distance unit

[jonjondev]

Currently, the ADSK plugin always authors metersPerUnit in the exported layer's metadata as so:

#usda 1.0
(
    defaultPrim = "pCube1"
    metersPerUnit = 0.01
    upAxis = "Y"
)

This is not compatible with some assumptions in AL's pipeline (we generally ignore Maya's units), and in continuing the effort to align the ADSK plugin to the AL plugin on static export behaviour, a new export option, -exportDistanceUnit (-edu), has been added to allow for disabling of this behaviour. By default it is set to true maintain current compatibility.