roll_median() produced incorrect results in the
presence of NAs. (#42)Properly document the align argument -- the function accepts
"center" rather than "middle". (#28)
Fixed an issue where empty fills were not handled correctly.
The interface has now been standardized such that each implemented window
function has version center-aligned by default (e.g. roll_mean()), a
left-aligned version (roll_meanl()), and right-aligned version
(roll_meanr()).
Implement rolling window functions for mean(), median(), min(),
max(), prod(), sum(), sd() and var().