Align and join a list of wide panels on their common dates. Each input
panel must have one Date column and a disjoint set of symbol columns.
join_panels(panels)A data.table with Date plus the union of all symbol columns, restricted to common dates.
List of wide panels (data.frame or data.table), each with columns Date + symbols.
All panels are first aligned to the intersection of their Date values.
Symbol names must be unique across panels; if the same symbol appears in
multiple inputs, an error is raised.