Zarr write + Zarr groups

[davidhassell]

Fixes #894 and #895

[davidhassell]

Not a very large PR, this one! I don't know what to do about the recipe files linting thing ...

[sadielbartholomew]

I don't know what to do about the recipe files linting thing ...

As for reverting the recipes linter/isort auto-changes as discussed in-person, the git command I mentioned that is simplest would be git checkout <commit hash> -- <dir> where you use the hash/reference of the commit where the recipes dir was in its original state, which I recommend as one just before the PR, and the docs/source/recipes path (apparently there is a git restore approach which is more modern but I don't have experience with that so I'll stick with what I've used and know to be good).

[sadielbartholomew]

I'll start reviewing now ignoring the linting changes to the recipes...

[sadielbartholomew]

All good, with tests all passing and relevant packaging, docs and tests updates for the new Zarr read and write funcionality - though ideally drop the recipes linting auto-changes as discussed in person (see my comment above for recommended command to revert).

[davidhassell]

Thanks for the git command. I did git checkout 9328936f04628183b8f6eda1a06006c50e5a54e9 -- docs/source/recipes/ which fixed 7 of the dodgy changes. I need to have a look at the others.