Calculate the number of specified periods in a given time series like data object.
Details
Essentially a wrapper to endpoints() with the appropriate period
specified. The result is the number of endpoints found.
As a compromise between simplicity and accuracy, the results will always round up to the nearest complete period. Subtract 1 from the result to get the completed periods.
For finer grain detail one should call the higher frequency functions.
An alternative summary can be found with periodicity(x) and
unclass(periodicity(x)).