lemmas about within-continuity

[affeldt-aist]

Motivation for this change

We'd like to propose the following lemmas about within-continuity.
They are being used in another development.
In within_continuous_comp, R : realFieldType looks generalizable.
Has anybody an idea?

Co-authored-by: @holgerthies

Checklist

• added corresponding entries in CHANGELOG_UNRELEASED.md

• added corresponding documentation in the headers

Reference: How to document

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