add setup_pet_forcing method

[hboisgon]

Issue addressed

Fixes #69

Explanation

For now just simple reprojection of PET without any complex correction when downscaling. This can be investigated in the future.

Checklist

• [x] Updated tests or added new tests
• [x] Branch is up to date with main
• [x] Tests & pre-commit hooks pass
• [x] Updated documentation if needed

[hboisgon]

Thanks for the review @dalmijn. What you mention is not really related to this PR is it?

In write_forcing, time_units is only used if freq_out is not None ie if you wan to split your forcing into several files when writing. Because in this case the user already needs to define manually freq_out then also defining the time_units should not be that big of an issue I think. If you write into a single file, xarray automatically uses the first time stamp of the data as the time_units but with several files wflow needs this to be common.

[dalmijn]

@hboisgon, I agree. I approve this PR then.