Test conversions

[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?

• Tests for xclim_modules/conversions.py
• Added context="hydro" to the precipitation conversion.
• Fixed a bug in tasmin/max_from_dtr when dtr was in different units than tasmin/max.

Does this PR introduce a breaking change?

• I changed dtr (the function) to dtr_from_minmax to avoid conflicts with dtr (the variable). This is only breaking if people directly used the Python file, which should be no one.

Other information:

[aulemahal]

Looks good!