ENH: Update NewNifti to use new znzlib API

znzlib is being updated to support more compression formats (see znzlib!3 (merged)) - as part of this there are some minor API changes. This MR updates NewNifti accordingly.

There are some minor API changes, but there are very few direct users of NewNifti so updating the rest of the code base should be straightforward:

• NiftiIO::fileIO::fileIO() bool use_compression parameter replaced with znz::znzFormat format.
• NiftiIO::saveImage bool useCompression parameter replaced with znz::znzFormat format.
• NiftiIO::fileIO::operator= changed from accepting a fileIO instance by value to accepting a fileIO rvalue