Update biopandas usage: Read pdb from list of strings

[dominiquesydow]

Code improvement in opencadd.io.dataframe:

Avoid to use the private PandasPdb method _construct_df. https://github.com/volkamerlab/opencadd/blob/912d4e98e89edf38707249fd4f034cea136e1932/opencadd/io/dataframe.py#L128

If possible in the future, use e.g.

ppdb = PandasPdb()
ppdb.read_pdb_from_list()  # New feature?

See feature request: https://github.com/rasbt/biopandas/issues/70

[dominiquesydow]

Wait for biopandas 0.3.0 release:

• Use new PandasPdb.read_pdb_from_list method
• Adapt to new file format error handling in read_pdb and read_mol2

(see biopandas CHANGELOG)