Add print warning if no natural isotopes when using add_element and wrote unit test

[cxtherineyu]

Description

When adding nuclides to a material with add_element, OpenMC will silently do nothing if there are no natural nuclides of that element (e.g., plutonium). It would be nice if the user encountered a warning about this, because if the user was trying to do this in the first place they did not realize/recognize that there weren't actually any natural isotopes for that element. I added a warning when this happens.

Fixes # 2926

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)

[gridley]

It appears this is a duplicate of the already existing #2938, so I'm going to close this. If I'm doing this by mistake, feel free to re-open!