add section on rechunking to chunk optimisation section of tutorial

[David-Baddeley]

Add some candidate rechunking docs as discussed in #727

TODO:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/tutorial.rst
• Changes documented in docs/release.rst
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)

[rabernat]

Thanks for this!

[shoyer]

nice!

[codecov]

Codecov Report

Merging #730 (e15fe20) into master (a8561fc) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #730      +/-   ##
==========================================
- Coverage   99.92%   99.92%   -0.01%     
==========================================
  Files          28       26       -2     
  Lines       10408    10048     -360     
==========================================
- Hits        10400    10040     -360     
  Misses          8        8              

Impacted Files	Coverage Δ
zarr/util.py
zarr/__init__.py

[joshmoore]

[ ] Add docstrings and API docs for any new/modified user-facing classes and functions

Ah... that feeling when it's all docstrings. 👍