Add ome-ngff metadata output to the zarr output of the fused images

[kgabor]

This ticket is to track the hackathon effort to make the multiview-reconstruction fusion zarr output ome (0.4) compatible.

• Add a simple ome-ngff .zattrs "top-level" metadata file.
• Pretty-print the json.
• Use subdirs instead of "." separator.

Development branch code is currently at AllenNeuralDynamics/PreibischLab-multiview-reconstruction.

[kgabor]

• Use gson annotations on MultiScaleImageto mark fields to be serialized. Do not use transient keyword which always prevents serialization. This could cause problems if instances will be used within Spark jobs.
• Add voxelsize info to ome-zarr output.