Correct openmc.Geometry initializer to accept iterables of openmc.Cell

[johvincau]

Description

In #3079, it was pointed out that the openmc.Geometry constructor should accept an optional object of either openmc.Universe or Iterable[openmc.Cell], but was coded only to accept the former.

Quick Fix

With PEP 604, we can change Optional[type] = None to the no-strings-attached alternate form type | None = None made available in Python 3.10+. Since | is also another way to express typing.Union[], it's easy enough to add the other needed case for openmc.Geometry.

In any case, in the rest of the codebase, the Optional[type] syntax appears 163 more times. It shouldn't be too difficult to refactor all of that into type | None = None for conciseness and readability, though that's probably for a different PR.

Checklist

• I have performed a self-review of my own code
• I have run clang-format (version 15) on any C++ source files (if applicable)
• I have followed the style guidelines for Python source files (if applicable)
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)

[paulromano]

Thanks @johvincau! If you are interested in making the change from Optional[x] to x | None elsewhere, that would be great but definitely should be a separate PR.