This is a method for the generic aggregate function.
The aggregate function was really designed for ts
objects, not tis objects which may or may not meet the
assumptions embedded in the function. The convert function is
better suited for tis series.
aggregate.tis calls as.ts on it's x argument,
then passes that and all other arguments on to aggregate.ts and
then turns the result back into a tis series. If there is a
local version of aggregate.ts that can be found by
exists("aggregate.ts", envir = globalenv()),
it will be used in preference to the function in package:stats.