Updated NURBS curve export for widths and knots attributes

[jonjondev]

This PR aims to continue the alignment between the AL and ADSK plugins' export functionality, updating the ADSK plugin's NURBS curve export of knots to avoid adding extra knots in non-wrapping curves (i.e. do not copy the first and last knots in the exported vector). The PR also adds a simple implementation for the widths attribute as lifted from the AL plugin's NURBS curve export translator in favour of just authoring a constant widths vector of [1].

[jonjondev]

Looks like it's building and passing tests. macOS failing with:

/lib/usd/translators/nurbsCurveWriter.cpp:205:53
error: lambda capture 'mayaKnotsCount' is not used [-Werror,-Wunused-lambda-capture]
        auto copyKnots = [&mayaCurveKnots, &curveKnots, mayaKnotsCount](size_t from, size_t to) {

Seems that Clang is just being fussy about warnings. I'll have a fix up in the morning (Sydney time), but should be reviewable.

[michaeljefferies]

The comment at the top says "updating the ADSK plugin's NURBS curve export of knots to avoid adding extra knots in non-wrapping curves (i.e. do not copy the first and last knots in the exported vector)", but according to the USD documentation there should be extra knots for both periodic and non-periodic curves: https://graphics.pixar.com/usd/dev/api/class_usd_geom_nurbs_curves.html#details