Ensure that chunking is respected

[RondeauG]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes #xyz
• (If applicable) Documentation has been added / updated (for bug fixes / features).
• (If applicable) Tests have been added.
• This PR does not seem to break the templates.
• CHANGES.rst has been updated (with summary of main changes).
  • Link to issue (:issue:number) and pull request (:pull:number) has been added.

What kind of change does this PR introduce?

• original_shape and chunksizes don't play well together. This PR makes sure that original_shape is always removed before saving a dataset.
• Also, (maybe new in the latest version of xarray and engine netcdf4?), it appears that dropping chunksizes leads to unexpected behaviours, such as bloated file size and incorrect chunking on disk. Thus, the chunksizes encoding was made more explicit.

Does this PR introduce a breaking change?

• No.

Other information:

Related Issues:
pydata/xarray#8385
pydata/xarray#8062