vchoi_fork

Merge pull request #2420 in HDFFV/hdf5 from revise_get_offset_1_12 to hdf5_1_12

* commit '9ac3f9526aeb0191eb36fc9eec4c63fa72a13697':
  Switch the 'get offset' operation from a dataset 'get' callback to a [native] dataset 'optional' operation.