Better save_to_zarr

[aulemahal]

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.
• HISTORY.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?

• New func xs.io.round_bits to round a floating point variable up to a number of bits, allowing for a much better compression afterwards.
• New "bitround" arg to save_to_zarr and save_to_netcdf, allowing to combine the bit rounding and the saving. The argument can take many forms for more flexibility.

Does this PR introduce a breaking change?

No.

Other information:

Default keepbits value is guessed from the variable name, with a default of 12 (0.0122% of rel error). Better guesses should be given in xs.io.KEEPBITS, but I have none for now.

[RondeauG]

Looks good, but I'll let @coxipi approve this, since this is his work.

[coxipi]

Nice, looks good to me!