Add a check for separation and tolerance in tweakreg

[mcara]

This PR adds a check that (abs_)separation > sqrt(2) * (abs_)tolerance input parameters to the tweakreg step. It also adds a comment in the docstring about these parameters.

Checklist for PR authors (skip items if you don't have permissions or they are not applicable)

• [x] added entry in CHANGES.rst within the relevant release section
• [x] updated or added relevant tests
• [x] updated relevant documentation
• [x] added relevant milestone
• [x] added relevant label(s)
• [ ] ran regression tests, post a link to the Jenkins job below. How to run regression tests on a PR
• [ ] All comments are resolved
• [ ] Make sure the JIRA ticket is resolved properly