[feature] Add frequency checks at specific timestamp

Rick van der Zwetrequested to merge
rixvet/python-crontab:new_frequency_at_timestamp into master

frequency_per_* functions does not take into account if running at specific timestamp, which is mildly confusing since it does not actual shows times the cron will execute at any given moment. The new frequency_at_* functions aims to fix this.

I was looking into a way of merging it into frequency_per_* functions without breaking backwards compatibility, how-ever I cannot seems to find a proper way. If you do believe this is overkill (feature creep), feel free to ignore.

Edited by Rick van der Zwet

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