Return complexity_limit in cyclocomp_linter to 15L

[sbfnk]

This was changed to 25L to accommodate the (possibly unavoidable) cyclomatic complexity of simulate_chains() introduced in #171

However, in light of the discussion in #179 I think it might be better to keep the limit at 15L and only exclude simulate_chains() by wrapping it in a # nolint start: cyclocomp_lintr and corresponding nolint end block.