Unable to export PDB DFs with model_id columns

[a-r-j]

Currently, PDB export breaks when model_id columns are added to the dataframe. This should be an easy fix by copying the dataframe and dropping any extraneous (i.e. non standard PDB colums) prior to parsing the output from the dataframe.

I also added a workaround that's needed when exporting (i.e., calling to_pdb() on) PandasPdb objects that have had a model_id column added to them. In the long term, I think it'd be good to have a fix merged into the master branch of BioPandas that types the model_id column as a str -> object column, but this workaround I've proposed should work for now.

Originally posted by @amorehead in a-r-j/graphein#309 (comment)