Print warning if no natural isotopes when using `add_element`

[aprilnovak]

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.

Alternatives

Print a warning if add_element does not add any nuclides.

Compatibility

New feature, better user experience.

[shimwell]

Perhaps we could add to the element.expand method that is called by material.add_element. We could add a check here in the expand method to check the length of isotopes found and print a warning if len(isotopes) == 0

perhaps something like

warnings.warn(f'No naturally occurring nuclides were found for element {self.name}')

[cxtherineyu]

Is this issue still open? I'd love to attempt to tackle it.

[shimwell]

I think the issue is all yours @cxtherineyu thanks for volunteering

[MicahGale]

My quick two cents. A custom Warning class should be defined so warnings can be correctly filtered by the end user instead of just calling Warnings.warn(""). I think maybe something like: NoNaturalIsotopesWarning