Add 'mathematics' tag to math exercises

[budmc29]

As discussed here: exercism/exercism#4110

[ZapAnton]

Two questions about this issue, if I may:

• luhn has the mathematics tag, even though it is not specified in the referenced issue, and it is core
  exercise, which contradicts the same issue.

• armstrong-numbers is a core exercise and has mathematics tag.

What should be done with these exercises?

[budmc29]

Can you ask the questions in the original issues please? I'm sure @kytrinyx will be able to answer those.

[NobbZ]

No, the questions belong here as the pr contradicts the linked issue.

Aside of that, no decision has been made yet. I'd not do premature PRs yet.

[budmc29]

I don't think I follow your first sentence?

[NobbZ]

It simply says, that asking those questions in the linked issue wouldn't make any sense, as they are specific to this PR. Espeically the luhn question.

[budmc29]

This is an opened issue, not a PR.
And I understand what you are trying to say.

We have a few exercises that already have the tag mathematics in our track, and in order to know what to do with them, it's best to ask the question in the original opened issue.

There might be other tracks that will have to deal with the problem, so it's better to have the discussion there, and also the exercises are general and not specific.

We'll work on a PR after the discussion will be finalized.