missing nifti support for writing aux_file meta data

Hi all,

For some reason the ability to set the aux_file meta-data for nifti files is not available. I expected the ITK writer to get the value from the meta-data dictionary but that is commented out. I assume there is a reason for not supporting writing this meta-data information?

Any insights appreciated.

Answering my own question: The support for writing the aux_file meta-data is added in this PR.