lag.time.table: Lag time.table
In rossklin/timetablr: Representation and basic operation for multiple time series.

Lags each time series by a number of time steps, optionally preserving the index/time values present in the data.table (in which case some information is lost).

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## S3 method for class 'time.table'
lag(tt, forward = TRUE, steps = 1L,
  preserve.frequency = TRUE, ...)

`tt`	time.table to lag
`forward`	Shift forward (default)
`steps`	Number of steps to shift (i.e. number of delta)
`preserve.frequency`	Preserve range (indeed, the exact values) of index/time values present in `tt`
`...`	unused Note that for now preserve.freuqency==FALSE is not implemented.