Extend statepoint dump feature to support dumps from the last N batches

[pranavkantgaur]

Description

This MR extends the statepoint dump feature as documented here to support dumping statepoints for last N batches. It is authored entirely using Copilot and reviewed by me.

Currently, the 'batches' key takes a list of batch numbers for which statepoint files should be saved, for instance: model.settings.statepoint = {'batches': range(5, n + 5, 5)}
this MR extends this to support configuration as:
settings.statepoint = {'batches': [10, 20, 30, -3]}, to write statepoints at specific batches and also keep the last 3 completed
batches (indicated by '-3').

Fixes #3630

Checklist

• I have performed a self-review of my own code
• I have run [clang-format](https://docs.openmc.org/en/latest/devguide/styleguide.html#automatic-formatting) (version 15) on any C++ source files (if applicable)
• I have followed the [style guidelines](https://docs.openmc.org/en/latest/devguide/styleguide.html#python) 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)

[yrrepy]

Like the initiative!

I think it should be it's own element, OpenMC tends to be more explicit than older codes that used switches and negatives to de-note different mechanisms.

something like this

  <state_point>
    <batches>10 20 30 </batches>
    <running_batches>3</running_batches>
  </state_point>

I personally would jettison the new tests, and I think the core code could be fair bit more compact.
The documentation is also too verbose.