Prepare for __array_function__ implementation in pint

[jthielen]

Pint is getting close to implementing the __array_function__ protocol (hgrecco/pint#764), which will allow many more numpy functions to work with and return pint Quantitys. However, this breaks many functions in MetPy that currently reattach units after assuming that units get stripped by the numpy functions. Would the best way forward be to

• Explicitly strip and reattach units in MetPy (by calling Quantity.magnitude or np.asarray(Quantity)) to maintain backwards compatibility?
• Bump minimum pint version to the release when __array_function__ is implemented and remove the strip-and-reattach workarounds?

(Also, just since it popped up in related issue suggestions, would this resolve what is left of #38?)

[dopplershift]

Good point. Really import to be proactive here so we don't have a release broken by this.