Add range-based partial fetch for h5wasm provider

[bmaranville]

Is your feature request related to a problem?

Reading very large files with the h5wasm provider is not possible, for several reasons:

1. maximum size of ArrayBuffer and also "file" in emscripten is often < about 2GB
2. maximum size of memory in the browser is a limitiation for in-memory file representation
3. unreasonable demands on network/infrastructure to download entire huge files.

Requested solution or feature

For web file servers with HDF5/NeXus files that support range requests, on-demand loading could enable access to very large NeXus files that would be infeasible to read as a whole, using emscripten's lazyFile functionality

Alternatives you've considered

HSDS and grove providers already allow this type of random access to parts of a NeXus file.

Additional context

Because sync file access is required, this might require refactoring the h5wasm provider to operate from a worker.
Note that it could potentially be refactored to a service worker that uses the same API as a grove server, if that simplifies things.

[bmaranville]

Note that for local files, the emscripten WORKERFS interface could be used to get random access to huge local files from a worker without copying the whole file into memory, which is another benefit of moving the provider to a worker.

[axelboc]

Note that it could potentially be refactored to a service worker that uses the same API as a grove server, if that simplifies things.

This would be brilliant! However, it seems that synchronous XHR requests inside Service Workers are currently not supported in Chrome and Safari—only in Firefox.