[Feature request] Return se, sd, ci etc. in `aggregrate.tracks` in addition to lower, upper. 

Currently, if I use `FUN="mean.se"` I get the columns `mean`, `upper`, `lower`. If I want to fit the data I need to use the `se` values as weights, would you consider adding them to the `aggregrate.track` `FUN` output? I can make a pull request if you want.