Fixing and revamping SceneCacheReader for Nuke

This pull request started as a bug fix (seen in the first commit). But when I've tried to create a unit test, using expressions in the file knob, I found more issues and it resulted in the second commit, which contains a collection of changes that improve the usability of the node ( allow TCL expressions in the file knob, keep selection after file changes - if applicable, hashes more reliable and should fix the issue with flickering geometry ). The third commit add some tests, including time remapping.