Update concatenation of pandas DataFrames

[Eeeeelias]

Newer versions of pandas don't allow for row-wise concatenation of dataframes using the .append method. This should fix this instance.

I did not have an example dataset that I could test this on, so I used a dummy set which worked.

I am not aware of any other instances where .append was used for dataframes, however I only checked functions.py.

[louadi]

Thanks for the fix!