Fixes crash when merging CortexPrimitives in Houdini 13.

Added merge constructor and fixed signature of stashed() for H13
Added IECore::EXRDeepImageReader and test cases.
Modified the SConstruct so that the EXRDeepImageReader is only built with OpenEXR versions that support deep images.
Changed the EXRImageReader to fail when trying to load a deep exr.
Added IECore::EXRDeepImageWriter and it's bindings and test cases.