Add unit helper for pandas data frames

[jrleeman]

I propose a helper that goes through the columns of a pandas data frame and returns a dict with the keys being column names and the values being united arrays. Of course this would only work on our data frames with a .units attribute, but it would reduce boiler plate in places.

[dopplershift]

You could pass in the units as a dictionary. Default it to None, and if not passed see if the data frame has a units attribute.

[jrleeman]

Closing in favor of this living in MetPy instead. See Unidata/MetPy#797