Returns the tangent effective Arps decline rate in units of [1 / to.period].
Details
The result should be applied as a tangent effective decline (see SPEE REP#6 [https://secure.spee.org/sites/default/files/wp-files/pdf/ReferencesResources/REP06-DeclineCurves.pdf]) specified in fractional (i.e. 95% = 0.95) units.
When appropriate, internally uses rescale.by.time to perform time unit conversion.