use xclim's cftime function

[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)
• [ ] 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?

• Fixes a bug introduced in #188.

Does this PR introduce a breaking change?

Other information:

cftime.datetime(2000, 1, 1, 0, 0, 0, 0, calendar='noleap', has_year_zero=True) breaks xr.infer_freq. cftime.DatetimeNoLeap(2000, 1, 1, 0, 0, 0, 0, has_year_zero=True) works.