Validates the data and index columns, sorts by unit then time, and attaches
"unit_var" and "time_var" attributes. Accepts either the new
unit_index/time_index arguments or the legacy index argument.
.make_panel(data, unit_index = NULL, time_index = NULL, index = NULL)A sorted data.frame with attributes unit_var and time_var.
A data.frame.
Character scalar: name of the unit identifier column.
Character scalar: name of the time identifier column.
Character vector of length 2 (legacy): c(unit_col, time_col). Ignored if unit_index and time_index are provided.