setup channel configuration to pass to aces_Writer

Currently, aces_writer only supports the writing of monoscopic images, so throw
an error if attempting to write stereoscopic images.
Add utility function to avoid issue with other places that
just define half to be an unsigned short instead of using
the ilmbase class
use utility function to convert to half instead of colliding
with the aces_writer definition of half which is just int16
format changelog
update headers for consistent license language across repostitory