[usdAbc] fixed reading indexed attributes

[AlexSchwank]

Description of Change(s)

• Indexed primvars use :vals suffix in ABC  #860 : removed :vals name suffix for indexed attributes
• Indexed primvars require int[] indices #859 : added support for indexed attributes using indices of type uint[] by converting to int[]
• updated testUsdAbcIndexedProperties baseline USD file

Fixes Issue(s)

• Indexed primvars use :vals suffix in ABC  #860
• Indexed primvars require int[] indices #859

[spiffmon]

I guess it's independent of this change, but this change in output hilites a problem we should probaly catch and fix: if alembic allows arbitrary properties to be indexed, we can't support that, since USD only allows indexing of primvars. So if we find anything other than an arbGeom property that's indexed, we should flatten it on the Alembic side?

[AlexSchwank]

I think this points to two more problems:

• in this case "vals" shouldn't be removed as there are no indices
• the xform attribute isn't really used in this prim

[jilliene]

Filed as internal issue #USD-5348

[cameronwhite]

I encountered similar issues and saw that PR #1635 already fixed the :vals suffix, but the int[] issue for indices still remains

@AlexSchwank - would you be able to rebase your changes? If you don't have time to work on this, I could also put together an updated PR