near_monotone_convergence

[IshiguroYoshihiro]

Motivation for this change

This is experimental generalization of monotone_convergence.

• a sequence which is nondecreasing not for all n but over some N.
• a sequence which is more than 0 not for all n but over some N'.

Checklist

• [x] 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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