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 (https://github.com/fortran-lang/stdlib/pull/771), this PR uses the shell to extract and compress npz files.

• [x] Load npz files.
• [x] Save npz.
• [x] Add tests.
• [x] Add examples.
• [x] Add documentation.

Related Issues: https://github.com/fortran-lang/stdlib/issues/486 https://github.com/fortran-lang/stdlib/issues/763