Imagecat 0.5.0

• Cryptomatte operator works with a wider range of files, and better matches the behavior of other implementations.
• Added an option to extract clown mattes from the Cryptomatte operator, with caller control over which mattes to include.
• Missing dependencies cause failures at run time instead of import time.
• Replaced the rgb2gray operator with dot, which can created weighted combinations of layers with any depth.
• Added new layer roles for RGBA, matte, luminance, depth, UV, position, velocity, and normal data, and subsets of RGB channels.
• Substantial reorganization and cleanup of the documentation.