Fix deprecation warnig in packing.py and replace .format() with f-strings where possible.

[chrisjonesBSU]

PR Summary:

This is a PR that does 2 things:

1. Fix a deprecation warning that is being raised about find_executable in distutils

>>> from distutils.spawn import find_executable
>>> find_executable("PACKMOL")
<stdin>:1: DeprecationWarning: Use shutil.which instead of find_executable

2. It also replaces .format() with f-strings in many places in an effort to clean things up a bit (I was bored at an airport)

PR Checklist

---

• Includes appropriate unit test(s)
• Appropriate docstring(s) are added/updated
• Code is (approximately) PEP8 compliant
• Issue(s) raised/addressed?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.34%. Comparing base (0ca30d2) to head (e120962).
Report is 7 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1192   +/-   ##
=======================================
  Coverage   87.34%   87.34%           
=======================================
  Files          62       62           
  Lines        6584     6584           
=======================================
  Hits         5751     5751           
  Misses        833      833           

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[chrisjonesBSU]

Merging this after discussing with Cal on a mosdef dev call.