Adding support for MaterialX namespaces declared in stdlibs

[vlasovi]

Description of Change(s)

• Make sure MtlxPlugin properly merges all stdlib files into a single library by calling 'importLibarary', thus preserving namespaces among other attributes
• Make sure HdStorm properly reconstructs MaterialX namespace name when assembling MaterialX document from HdMaterialNetwork2

Fixes Issue(s)

• Partially addresses Support for MaterialX namespaces in mtlxPlugin and HdStorm #1614

[jilliene]

Filed as internal issue #USD-6927

[spiffmon]

Thanks, @vlasovi - it looks like we do not have a signed CLA from you, which we would need to be able to consider this request. Could you submit one?

[vlasovi]

@spiffmon I'm a developer at Autodesk and I believe that Autodesk has already signed a corporate CLA with you. Do you want me to sign an individual contributor CLA as well?

[spiffmon]

Ah, in that case, please contact Gordon and/or Ian to get you added to the list of approved Autodesk contributors - thanks, @vlasovi !

[vlasovi]

@spiffmon, I believe that Gordon added me to the CLA.

[spiffmon]

Yes, you're all set on the CLA front, @vlasovi . While it's not yet possible for you to add an imaging test, would you please add a test in usdMtlx that verifies the handling of namespaces, and also update the overview.dox to remove namespaces from the list of unsupported features?

Thanks!

[vlasovi]

@spiffmon I updated the PR with the requested changes.