Sorgenfrey line and properties

[motikaku]

Motivation for this change

This PR demonstrates separation axioms.
It defines the Sorgenfrey line and proves it totally disconnected.
We intend to also prove that it is T6.
This topology is mentioned by @zstone1 in the discussion below.

#677 (review)

Coauthored-by @garrigue @t6s .

Checklist

• added corresponding entries in CHANGELOG_UNRELEASED.md

• added corresponding documentation in the headers

Reference: How to document

Merge policy

As a rule of thumb:

• 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.

Reminder to reviewers

• Read this Checklist
• Put a milestone if possible
• Check labels

[garrigue]

BTW, T6 is mentioned in the header of separation_axioms.v, but there does not seem to be a definition for it.