Turc for negative Tmean close to 15 gives strange PET value

[raoulcollenteur]

Describe the bug
I encountered an issue when using Turc to compute the PET for the following values:

idx = pd.to_datetime(["2000"])
rs = pd.Series(5.0976, index=idx)
rh = pd.Series(76.9, index=idx)
tmean = pd.Series(-15.1, index=idx)

pyet.turc(rs=rs, rh=rh, tmean=tmean)

> 337.10734054400115

To Reproduce

Run example above.

Expected behavior

I guess PET should simply be zero? Cut of for when tmean < 0, which I think is recommended for Turc.