at_interval() function

This would be a generalization of on_second() or on_millisecond() where one can specify
the interval and maybe the unit:

on_interval (360000, ms) or on_interval (360000ms) =
on_interval (360, s) or on_interval (360s) =
on_interval (6, m) or on interval( 6m) =
on_interval (0.1, h) or on_interval (0.1h)

The default value can be 1, and the default unit can be seconds, so on_interval() will run ever second, and on_interval(h) will run every hour.