Fixes and features for "deep" images

1. Fix to OpenEXR reading and writing to rename or suppress certain exr2 attributes that should be set by the library itself (chaos results from absentmindedly passing them from a read file to a written file, if it doesn't match what the written file should really be).

2. Implement "flatten" operator that depth composites the samples within each pixel of a deep image, to yield a flat image.
  a) ImageBufAlgo::flatten()
  b) oiiotool --flatten
  c) Python ImageBufAlgo.flatten
  d) testsuite cases & docs
This is currently not fully correct... as written, flatten only works properly for deep files whose samples are sorted and non-overlapping ("tidy", in OpenExr nomenclature).  I will need to return to this later to make it robust for overlapping or unsorted samples, but for the time being, I am only generating tidy files and need this feature committed.