hdf5_msb

Merge pull request #2452 in HDFFV/hdf5 from ~JAKE.SMITH/hdf5:feature/vfd_splitter_mirror_a to develop

Add Splitter VFD and Mirror VFD.

* commit '664fc6af47e369152727ff66d68d8532feb0e210':
  Add enable-mirror-vfd flag to configure options.
  Tidying of Mirror VFD.
  Add function header comment for `mirror_writer:run_writer()`.
  Add Splitter VFD to library.