Use sdba options for xclim by default

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.
[x] 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?

Instead of setting the options only when calling xclim's adjust, we set the options globally at import time.

This fixes the subtle issue noted in Ouranosinc/xclim#1674 : the context options are not seen by all code when dask-backed data is used. as the actual computation occurs outside of the context.

It also allows to change these options at run time. xc.set_options(sdba_encode_cf=False) will fix #362, for example. This was not possible earlier as the option context was within the function and couldn't be modified by the user.

Does this PR introduce a breaking change?

Yes, issue #362 will now occur on dask arrays too, until xclim's PR is merged. But it can be circumvented as shown above.

Ouranosinc / xscen