Fix subdivision when primvars are set in parent primitives

[sebastienblor]

Changes proposed in this pull request
The test done by HasAuthoredAttribute that is used to properly set subdivision attributes, is not seeing the primvars authored in parent primitives. I replaced these calls with a new function HasConstantPrimvar that looks for this primvar name in the context list of primvars.
I considered different approaches, for example extending HasAuthoredAttribute to invoke itself recursively on parent primitives.
However, it would be a bit expensive to search for an attribute based on its name (i.e. with a map search) through the whole hierarchy and for each node.

Here, even though we're doing a linear search over the list of primvars, it should be more efficient as TfTokens are fast to compare.

I also saw a few occurrences of HasAuthoredValue on attribute smoothing to enable it by default. This shouldn't be needed anymore since smoothing is now enabled by default in arnold polymeshes

Issues fixed in this pull request
Fixes #1982