data.table: inplace option + group_by does not reorder data.table

- inplace:
  - inplace option re-implemented for mutate, mutate_each
  - inplace option created for arrange using setorder(na.last=TRUE)
  - inplace option created for group_by
- group_by does not reorder a data.table anymore. I just removed the setkeyv line since I could not see why it mattered.