change of sigma-algebra

[affeldt-aist]

Motivation for this change

This PR proposes a change of default sigma-algebra for real numbers (from ocitv to open sets) so that @Brixfoly can easily further generalize to normed modules.

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

[affeldt-aist]

@Brixfoly @hoheinzollern this PR should be almost clean and ready to support experiments with normedModType

[affeldt-aist]

PR #2017 provides evidence that this current PR (2016) is useful to generalize to sigma-algebras over normed modules.

[affeldt-aist]

Since it has been shown useful for PR #2017 (and therefore tested), written by three people, it does not break the CI, we will merge soon unless somebody objects.