unit redefinition help for /s

[kgoebber]

Description Of Changes

The issue has arisen with older model output data where the stored vorticity unit is /s, which will then fail on any action with a unit with the error pint.errors.DefinitionSyntaxError: missing unary operator "/". This solution adds a small fix in the units module to replace /s with s**-1. An example dataset that this occurs with is older GFS data available through the RDA archive.

Minimum example to produce error:

from datetime import datetime

from metpy.units import units
import xarray as xr

ds = xr.open_dataset('https://rda.ucar.edu/thredds/dodsC/aggregations/g/ds083.2/1/TP')

ds.Absolute_vorticity_isobaric.metpy.sel(time=datetime(2000, 1, 4, 12), vertical=500*units.hPa).metpy.quantify()

[dopplershift]

Thanks for fixing it up! I think I have a somewhat simpler solution for the pre-processor.