Warning in pandas tests #187
Description
The recent changes in #174 and #184 seem to have introduced the following warning when calling the tests:
tests/pandas_test.py::test_nan_df_export_import
/home/fjosw/Codes/pyerrors/pyerrors/input/pandas.py:85: UserWarning: nan value in column int will be replaced by None
warnings.warn("nan value in column " + column + " will be replaced by None", UserWarning)
Can you please have a look at this @jkuhl-uni ?