hpfilter — Hodrick Prescott filter
[hpy]=hpfilter(y,lambda)
y = either
* a time series, or
* a real (nx1) vector, or
* a string equal to the name of a time series or a (nx1) real vector between quotes
lambda = the smoothing parameter
hpy= the smoothed filtered series of the same type than y (if y is not a string) or evstr(y) (if y is a string)