isolated points

[affeldt-aist]

Motivation for this change

fixes #1775

define isolated points and properties and refine a lemma about closure

@IshiguroYoshihiro

Checklist

• added corresponding entries in CHANGELOG_UNRELEASED.md

• added corresponding documentation in the headers

Reference: How to document

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• PRs with several commits that make sense individually and that
  all compile are preferentially merged into master.
• PRs with disorganized commits are very likely to be squash-rebased.

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[affeldt-aist]

The last commit adds a proof that there is a countable number of isolated points, this is need for a proof by @IshiguroYoshihiro

[affeldt-aist]

It looks like the introduction of isolated points allowed for a strict improvement of the lemma closure_isolated_limit_point since it replaces a union with a disjoint union and we also have a used for the new lemma countable_isolated (work by @IshiguroYoshihiro ). Do you (@zstone1 , @t6s ) think we are missing something?
(Also, CI errors are unrelated.)

[t6s]

Looks good, modulo the refactoring comment.