corrected DTR ceiling boolean expression and tests accordingly

[emileten]

This corrects a bug introduced by #165.

In the DTR ceiling we were making use of direct comparison operators like or etc... Which lead to a typical error with arrays.

This was not caught by the tests because there was only one latitude coordinate.

Changes :

• switched to xr.ufuncs logical operators
• enhanced tests by using arrays with with several latitude coordinates.

I am not doing anything to the changelog because this is just an extension of #165.

[brews]

Wicked. Thanks @emileten. Appreciate you updating the test for this. That's the right move.

Your instinct about not adding a new CHANGELOG entry for this is correct, instead, just add this PR to the existing entry. I added this for you in 20e923e.

[emileten]

Thanks for doing that @brews.