Support for Updraft Helicity

[jthielen]

What should we add?

Updraft helicity (vertical integral of vertical velocity times vertical relative vorticity) is a commonly used field in high-resolution NWP applications. In comparing wrf-python's set of diagnostics against what can be calculated with MetPy, UH was the only diagnostic that was clearly within scope for generalized use in MetPy yet did not have an existing or planned implementation.

This is a fairly straightforward calculation, the main problem will be handling the limits of integration (which are typically specified in height AGL) for arbitrary vertical coordinates (which overlaps with some other calculations currently reliant on get_layer, xref #1533 (comment)).

Reference

Kain, J. S., Weiss, S. J., Bright, D. R., Baldwin, M. E., Levit, J. J., Carbin, G. W., Schwartz, C. S., Weisman, M. L., Droegemeier, K. K., Weber, D. B., & Thomas, K. W. (2008). Some Practical Considerations Regarding Horizontal Resolution in the First Generation of Operational Convection-Allowing NWP, Weather and Forecasting, 23(5), 931-952. Retrieved Nov 18, 2021, from https://journals.ametsoc.org/view/journals/wefo/23/5/waf2007106_1.xml