Check input of composition

[mantepse]

Fix #14862

[github-actions]

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[mantepse]

@fchapoton, I think that this is an improvement, so let's get it in.

In the longer run I'd really like to make Composition only accept positive integers, and use IntegerVector for weak compositions (perhaps WeakComposition should be an alias). However, this can be done incrementally, I am currently doing it for tableau.py.

[mantepse]

Looks good to me. Perhaps we should have a check argument to bypass the checks, but I don't think it is necessary right now.

[mantepse]

@fchapoton, I just realised that I cannot approve this formally, because I created the pull request...

[fchapoton]

I think it would be good to have Travis' opinion on that. @tscrim what do you think ?

[tscrim]

I think it would be good to add a check to the __init__, but I wouldn't make a requirement for setting this to a positive review.