Parallelise IO

[clbarnes]

Probably in a separate crate depending on rust-n5, include a ParallelN5Ndarray(Reader|Writer) which parallelises the functions available in https://github.com/aschampion/rust-n5/blob/master/src/ndarray.rs#L121 , using https://github.com/aschampion/rust-n5/blob/master/benches/parallel_write.rs#L69 as an example.

[aschampion]

Once async lands in stable rust (next release), I'll probably promote the AsyncN5Reader traits from n5-wasm to rust-n5. This could make the the separate crate pretty trivial.

[aschampion]

See also https://github.com/aschampion/n5gest/blob/master/src/main.rs#L371-L464 for how to use a thread local cache when reading to reduce allocations.

[clbarnes]

Partially implemented (python-side)