Review: Updated Nuke OCIODisplay, added config.getCacheID(), context.getCacheID(), ClearAllCaches()

Jeremy Selan <jeremy...@...>

nuke ociodisplay: ocio_populate_viewer works again
nuke ociodisplay: Renamed viewer knobs to "gain" and "gamma". This
allows them, when used as ViewerOps, to be driven by the existing
monitor interface.
nuke ociodisplay: added prototype for channel swizzling support. This
can't be enabled though until the OCIO cpu path supports alpha channel
handling (otherwise 'alpha' visualization wont be supported)
nuke ociodisplay: Added context support (per shot support)
core library: configs, and contexts can report cacheIDs.
Added OCIO::ClearAllCaches(), which will drop all cached information
about files on disk. (Will cause luts to be reloaded on next use)

closes GH-46
closes GH-15

-- Jeremy