Fix for two callbacks defining the same custom attribute

Previously, the attribute would appear twice in attributeNames(). It would get the value from the first callback regardless, but the second attempt to write the value (in the ROP) throws because SceneCache does not accept duplicate sample times for a single attribute.
Fixed exception when trying to read attributes that didn't exist in the underlying scene.
Replacing hasAttribute with try/catch.

Also added todo about performance issues in previous commit
Merge pull request #197 from andrewkaufman/ropAttrFixes

HoudiniScene custom attribute bug fixes