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
fix potential crash accessing de-allocated temporary memory
switch the half type in aces container to half_bytes

Allows us to remove the hack function in format to get around the
collision between the half class and the typedef to 16-bit integer
change half_bytes to halfBytes
update call to aces_write to match current declaration

issue #23:
There is a second definition of aces_write in the #else clause of aces_file.cc
Change the "const void *pixels" to "const float *pixels" to fix this.