ENH: Load image from URL

[effigies]

We should think about how to fetch data remotely, and ideally maintaining transparent random access.

@yarikoptic Pointed me at fsspec, which would be a good start. Gzip indexing is going to be a big issue. I'll cc @pauldmccarthy in case he has thoughts/experience on here.

Got a pointer to Neuroglancer. I think their NIfTI is all Javascript, but there might be something to learn over there.

[pauldmccarthy]

Sorry, just saw this mention - recent changes to indexed_gzip added support for Python file-like objects, so adding support for remote files will hopefully not be too painful (with the caveat that a pre-built index needs to be provided, or that reading needs to start from the beginning of the file).