Do not count zero energy pulse heights

[GuySten]

Description

Currently openmc tally zero energy pulse heights.
This PR fix that by skipping zero energy pulse heights.

Checklist

• I have performed a self-review of my own code
• I have run clang-format (version 18) 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)

[cfichtlscherer]

The change makes sense to me, physically a detector doesn't register a "zero pulse," and it brings the PHT in line with how every other tally in OpenMC behaves (no event -> no score). That said, there is an implicit PHT convention coming from MCNP's F8 tally. MCNP as well keeps zero-energy events and expects the user to partition them out by adding a zero bin and an epsilon bin (~1e-5) on the E card (see Booth, LA-13955). That's the motivation for the current design choice: expose the events, let the user handle them.

So I lean towards keeping it as it is. If we do this design change, some other things to consider:

• We should be explicit about this divergence from the F8 convention.
• Use <= instead of ==
• We should also update the example:
  https://github.com/openmc-dev/openmc-notebooks/blob/develop/gamma-detector.ipynb

[GuySten]

In the example you linked there is no epsilon bin.
So if we do not do this change we will have to update the example.

I lean towards not scoring zero pulse events.
I do not understand why someone will want to track zero energy pulse events.

We could also decide to ignore pulses under 10eV so we will not need epsilon bin.

@paulromano, what do you think?