writing example of authoring variants with reference

[ankmachine]

FIxed the bug for #58

VariantSetNames and variantSelection are store under internal names that differ from the USD text grammar keywords.

Also, writing of example of how to author variants which are referencing different prim from another usda file. In this case scene.usda is referencing prims "/BlueSphere", "/GreenSphere", "/RedSphere" from sphere.usda

run code

cargo run --example author_variant_and_reference

scene.usda

#usda 1.0
(
    defaultPrim = "ColoredSphere"
)

def "ColoredSphere" (
    prepend variantSets = "color"
    variants = { string "color" = "blue" }
)
{
    variantSet "color" = {
        "blue" (
            prepend references = @./sphere.usda@
        ) {
        }
        "green" (
            prepend references = @./sphere.usda@</GreenSphere>
        ) {
        }
        "red" (
            prepend references = @./sphere.usda@</RedSphere>
        ) {
        }
    }
}

sphere.usda

#usda 1.0
(
    defaultPrim = "BlueSphere"
)

def Sphere "BlueSphere"
{
    color3f[] primvars:displayColor = [(0.0, 0.0, 1.0)]
}

def Sphere "GreenSphere"
{
    color3f[] primvars:displayColor = [(0.0, 1.0, 0.0)]
}

def Sphere "RedSphere"
{
    color3f[] primvars:displayColor = [(1.0, 0.0, 0.0)]
}

[mxpv]

Thanks!