IECoreNuke::LensDistort Refactor.

Completely refactored the IECoreNuke::LensDistort node to remove caching issues introduced by nuke when using dynamic knobs. The node now creates a static like of knobs which are renamed and hidden/revealed dynamically. 

Added a "black outside" option which changes how pixel samples out of the image data window are handed. When enabled, the black-outside option samples black pixels rather than clamping the sample coordinate. 

Added an option which allows the user to specify how missing cob files are handled. The user can choose between an error being displayed or the use of the nearest frame's cob file.