fix: Allow for shutil.copytree to have existing dirs

[matthewfeickert]

• Add dirs_exist_ok=True to allow the copying operation to continue if it encounters existing directories.
  • c.f. https://docs.python.org/3.12/library/shutil.html#shutil.copytree
  • dirs_exist_ok was added in Python 3.8.
• Amends PR #155.

* Add dirs_exist_ok=True to allow the copying operation to continue if
  it encounters existing directories.
   - c.f. https://docs.python.org/3.12/library/shutil.html#shutil.copytree
   - dirs_exist_ok was added in Python 3.8.
* Amends PR https://github.com/recast-hep/recast-atlas/pull/155.