• Closes #2217
• Tests added
• Passes isort . && black . && mypy . && flake8
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst

Reading #2217, I've implemented fast algorithms for union and intersection of arrays with numerical tolerance.
This works fine in the "normal" case, when each array has all its values different (outside the tolerance) and the first and second arrays have some values in common within the tolerance. Conversely, the behavior is not well defined when one array has some values that are within the tolerance (or are equal) of each other. In this case, the behavior of union and intersection is not well defined anyway. The "bad" cases could be checked to raise an Exception (duplicate values within the tolerance), but this is not implemented yet.

I've also implemented a function to test index equality within the tolerance.

At last, the logic of xarray.align has been changed to deal with the tolerance. It was not possible to avoid some changes in align.

I'd appreciate some tests and code review, because there are certainly some rough corners I've not though about.