transform.for.growth.analysis: Transform data set for analysis in growth regressions
In skranz/sktools: Helpful functions used in my courses

Generate growth rates, absolute gains, average values, first and last values in time windows of size ahead+1

## S3 method for class 'for.growth.analysis'
transform(dat, ahead = 1,
  trans.vars = names(which(sapply(dat, is.numeric))),
  fixed.vars = setdiff(colnames(dat), trans.vars),
  slide = ifelse(overlapping, 1, ahead), overlapping = TRUE)

`dat`	a data frame increasingly sorted in time with no gaps
`ahead`	for how many periods ahead shall growth rates etc be computed
`trans.vars`	variables that shall be transformed . By default all numeric variables in dat
`fixed.vars`	variables whose first value in each time window that shall be added to the results without any transformation.
`overlapping`	(default=TRUE) shall time windows be overlapping or not
`slide`	the distance of window starts is by default 1 if overlapping == TRUE and ahead if overlapping==FALSE