adding get_all_nuclides method to geometry class

[shimwell]

Description

I've been using openmc.deplete.get_microxs_and_flux(nuclides =...) and wanting to pass in the nuclides present in the geometry. Having the proposed simple get_all_nuclides method on the geometry class would save me some user script inconvenience. Getting a mutable flat sequenceof all the nuclides without duplicates can certainly be done on the user script it is just a mild inconvenience.

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]

@shimwell Thanks for the PR here. I just updated it to improve the efficiency (using a set rather than a list while accumulating the nuclides).