fix bug in issue 1635

[kgoebber]

Description Of Changes

This PR is a fix to the bug reported in issue #1635. The issue revolved around the use of a temperature variable in generating a weighted mean value. The fact that the np.trapz function now accepts unit arrays this fix simply allows the units to pass through the function and yield proper units at the end instead of stripping and reapplying units.

A test has been added to account for using temperatures to compute a mean weighted value. The input is in Celsius with the output in Kelvin.

The only question, which I raised in a comment on the issue, is whether we desire to have the output come back this way, or force it back to the input units. The current behavior in this PR is similar to that of the parcel_profile function.

Checklist

• Closes mean_pressure_weighted unit behavior #1635
• Tests added
• Fully documented

[dopplershift]

I think we can leave the return as is--I like the idea of us doing no additional work. I don't consider return units part of the API (only correct dimensionality, and really returned units are dictated frequently by our upstream dependencies), so we can always change if there's a more convenient default behavior.