bigdataviewer-n5

This adds a BigDataViewer ImgLoader backend for the format bdv.n5.cloud.

For example something like this XML would use it:

<SpimData version="0.2">
  <BasePath type="relative">.</BasePath>
  <SequenceDescription>
    <ImageLoader format="bdv.n5.cloud" version="0.1">
      <n5 type="relative">dataset.n5</n5>
    </ImageLoader>
    ...
  </SequenceDescription>
  ...
</SpimData>

In contrast to the bdv.n5 format in bigdataviewer-core, the bdv.n5.cloud backend uses N5Factory to construct a N5Reader. The bdv.n5 format is (was) hard-coded to use a filesystem reader and N5 format, so it will not work for N5 stored on AWS S3 or Google Cloud, and/or in Zarr format.

However, the handler for bdv.n5 will now try to forward to bdv.n5.cloud if opening with the hard-coded reader fails. Hopefully, there should be no need to specify bdv.n5.cloud in the XML. It should be sufficient to put the bigdataviewer-n5 artefact on the classpath.

The bdv.n5.cloud format is probably temporary, to be removed when we figure out how to do this properly. Avoid using it explicitly if possible.