Handle npz files

[minhqdao]

Adds handling (loading and saving of npz files).

As an npz file is nothing but npy files zipped together to a single file, we either need to include a library that handles zip file creation/extraction for us or call system programs via the command line. In contrast to a previous attempt (#771), this PR uses the shell to extract and compress npz files.

• Load npz files.
• Save npz.
• Add tests.
• Add examples.
• Add documentation.

Related Issues:
#486
#763

[awvwgk]

Thank you for this PR, this introduces a lot of needed features in stdlib. However, it is hard to review because it introduces three new features to stdlib, which are dependent but each should have their own independent review.

It would be great if you could open a PR for the filesystems first, a PR for the zip support second and finally a PR for the npz support.

Having dependent PRs is a bit challenging for reviewing. One option would be to push the three branches to the stdlib repo and open PRs from within the stdlib repo, as this improves the experience of reviewing dependent branches. Alternatively, you can open PRs from all three branches from your fork and we try to focus the review only on the parts introduced by the respective PR.

Please let me know if you need any support with this.