Windchill calculation does not work with Kelvin as input units

[jrleeman]

This should work, but doesn't (full trace included).

mpcalc.windchill(263 * units.kelvin, 20 * units('m/s'))

```python --------------------------------------------------------------------------- OffsetUnitCalculusError Traceback (most recent call last) in () ----> 1 mpcalc.windchill(263 * units.kelvin, 20 * units('m/s'))

/Users/johnleeman/anaconda/envs/unidata-workshop/lib/python3.6/site-packages/metpy/units.py in wrapper(*args, **kwargs)
204 ' x = x * units.meter / units.second')
205 raise ValueError(msg)
--> 206 return func(*args, **kwargs)
207
208 return wrapper

/Users/johnleeman/anaconda/envs/unidata-workshop/lib/python3.6/site-packages/metpy/calc/basic.py in windchill(temperature, speed, face_level_winds, mask_undefined)
159 delta = temperature - 0. * units.degC
160 wcti = (13.12 * units.degC + 0.6215 * delta -
--> 161 11.37 * units.delta_degC * speed_factor + 0.3965 * delta * speed_factor)
162
163 # See if we need to mask any undefined values

/Users/johnleeman/anaconda/envs/unidata-workshop/lib/python3.6/site-packages/pint/quantity.py in add(self, other)
585
586 def add(self, other):
--> 587 return self._add_sub(other, operator.add)
588
589 radd = add

/Users/johnleeman/anaconda/envs/unidata-workshop/lib/python3.6/site-packages/pint/quantity.py in _add_sub(self, other, op)
574 units = other._units
575 else:
--> 576 raise OffsetUnitCalculusError(self._units, other._units)
577
578 return self.class(magnitude, units)

OffsetUnitCalculusError: Ambiguous operation with offset unit (degC, kelvin).

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