DAOS-VOL

Merge pull request #21 in HDF5VOL/daos-vol from ~JHENDERSON/daos-vol:master to master

* commit '1e670b05927223fc7c1a40e192c9f78876d924a3':
  Revise H5Aexists(_by_name) implementation to not use daos_obj_list_akey
  Update User's guide for H5Arename/delete/get_info
  Implement H5Arename(_by_name)
  Refactor attribute-specific callback into smaller functions
  Implement H5Adelete(_by_name)