Added ability to use deep data in EXR files

The interface for deep files is similar to regular files, the differences are:
- It's necessary to set data type with ImfHeaderSetType() function.
- It's necessary to output deep data with ImfOutputSetDeepFrameBuffer()
function.
Created deep exr file will be of OpenEXR2.0 format.

The following compression schemes are the only ones permitted for deep data:
- NO_COMPRESSION
- ZIPS_COMPRESSION
Added sample file that generates simple gradient with deep data