hdf5_der

Merge pull request #197 in HDFFV/hdf5 from ~DEROBINS/hdf5_der:cache_image_partial to cache_image

Merge fix for parallel failure.

* commit 'ad37ae2097374d34c80a1715c3ca213e09da26de':
  Add a flag for H5C__flush_single_entry() to allow the parallel code to request that images for entries be generated, even when not writing the entry from an MPI rank.