added stable and unstable nuclides to the Chain object

[shimwell]

Description

I find myself wanting to access the stable and unstable nuclides in a chain file when post processing results. Perhaps this is functionality useful for others. It doesn't save much effort but could perhaps still be useful. I tend to filter the material isotopes to see which nuclides produced are unstable, then I further filter by activation, decay heat etc when producing plots of important isotopes in activated material as a function of time.

I was not sure if it is best to return a list of Nuclides or list of strings (nuclides names) but I went for the list of nuclides as the Chain.Nuclies also returns a list of openmc.deplete.Nuclide

Fixes # (issue)

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 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 @shimwell. I changed them each of them to a @property. Is that cool with you?

[shimwell]

That is certainly cool with me. Thanks

[shimwell]

Enabled auto merge