[Biopython] saving multiple pdb files to the same output pdb file

[Fomightez]

Because I don't have time at the moment to test my old code and update
to Python 3, I contacted Artemi off list and pointed him at the script
`merge_multi_PDBs_into_single_file.py` described and available at
https://github.com/fomightez/structurework/tree/master/python_scripts
. I developed it based on discussions by Claudia Millán Nebot , Hongbo
Zhu, and Joso Rodrigues on the user's list in 2015. Claudia Millán
Nebot was nice enough to let me use her function at the core.

Noting it here as well in case it helps anyone.
I'll try to test it in Python 3 (and do any necessary updating) and
add a Binderized Jupyter notebook demo there too soon.

(In case, some folks don't know about the Binder system yet, you can
see an example in action at
https://github.com/fomightez/patmatch-binder . Just go there and click
on the `launch binder` badge.)

-Wayne