HoudiniScene default time

Since HoudiniScene needs access to SOP data to fully traverse they hierarchy, it was cooking those SOPs during non-time dependent methods, like hasObject and childNames. It had been using CHgetEvalTime() do perform these cooks. This was resulting in double cooking when evaluating a time other than the current UI time (as in a ROP caching context).

This pull request adds get/setDefaultTime methods, which provide a default cooking time for methods that do not accept time as an argument (e.g. hasObject or childNames). The default time will pass through to children automatically. If left unset, CHgetEvalTime() will be used for these queries.