changing y axis label for heating plots

[shimwell]

While plotting some heating cross sections we noticed that the units on the Y axis were in barns when perhaps they should be in eV/collision or something else.

This PR adds special handeling if the heating or heating-local types are plotted.
It also checks that the cross sections are not mixing heat with non heat cross sections as these can't share a Y axis units

I've added tests for the new use cases

Here is a before and after screen shot

sample code for these plots

import matplotlib.pyplot as plt
import openmc

fig = openmc.plotter.plot_xs(
    reactions={
        "Be9": ["heating-local"],
        "Li6": ["heating"],
    },
)
plt.show()

tagging @rlbarker for your interest

Checklist

• I have performed a self-review of my own code
• I have followed the style guidelines for Python source files (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)

[giovanni-mariano]

Hello Jon,
thank you for fixing this: I think the unit should "eV x barn".

Best,
Giovanni

[shimwell]

Just to mention if this PR is successful then I would be keen to follow up with a PR that fixes the axis label when MT 444 damage is plotted. I think the units for the 444 reaction units are eV barn?

[shimwell]

Just wondering if anyone has had time to review this an if there are any changes needed.

[paulromano]

Thanks for the ping @shimwell. Will take a look soon

[paulromano]

I made a small fix to add the "damage-energy" score, which is also eV-barn. Good to go now; thanks!

[shimwell]

Thanks for adding damage energy. That subset change is also better than the way I was doing it. Many thanks for the review.