dt_sum: Data.table Summen von Variablen
In ycphs/data.table.xlsx: Summarize data.table and safe them as xlsx

creates a sum dependend of variables

1	dt_sum(dt, ind_var, dep_var, dep_on, bylist, na.rm = TRUE)

`dt`	Datatable
`ind_var`	indipendent variable vector index or names
`dep_var`	dependent variable vector index or names
`dep_on`	dependent on varibale name or index one or per dependent variable
`bylist`	list of group by elements
`na.rm`	handles the na.rm variable at summing