python3 is now default intepreter for .py executables

I replaced

#!/usr/bin/env python

with

#!/usr/bin/env python3

...everywhere.

This means users are no longer required to use "python" instead of "python3" when trying to run .py executable files directly from the shell (such as mol22lt.py, ltemplify.py, and genpoly_lt.py). This will enable most users to be able to invoke these python programs directly by their names, without having to predicate them with "python3".

Comment:

I'm sorry I didn't make this change earlier. I've been using anaconda python for a long time, and anaconda automatically adds a link from "python3" to "python", so that users can type "python" instead of "python3". I didn't realize that for most users, "python" isn't even defined.