Fix Py34: Iterate over datetimes

The old pandas versions available for Py34 cannot subtract timedeltas
from ndarrays. Subtracting them individually works and was used in the
fix for later pandas versions:
pandas-dev/pandas#22054

Author: st-bender

src/nrlmsise00/dataset/core.py

@@ -233,7 +233,7 @@ def msise_4d(
 	sw = sw_daily()
 	# convert arbitrary shapes
 	dts = np.vectorize(pd.to_datetime)(time, utc=True)
-	dtps = dts - pd.to_timedelta("1d")
+	dtps = np.array([dt - pd.to_timedelta("1d") for dt in dts])
 
 	ap = _check_gm(ap, dts, df=sw[["Apavg"]])
 	f107 = _check_gm(f107, dtps, df=sw[["f107_obs"]])