More directly support more displacement field file formats

[bogovicj]

export

We may be able to rely on BioFormats for exporting to nrrd/nifti and just let BigWarp export displacement fields to ImagePlus.
We could also provide converters (see below) from the saalfeldlab-(hdf5/n5) format BigWarp can currently export to directly.

import

We could import nrrd / nifti files that contain deformation fields directly for importing, but its easier to convert instead.

conversion

rather we can provide plugins that convert nrrd / nifti to and from saalfeldlab-(h5/n5).