average_spc() : add further argument check for argument by (list-column not supported)

[philipp-baumann]

average_spc(spc_tbl, by = "group_id", column_in = "spc"),

data.table::setkey raises an error downstream in the call stack if by is of type list (list-column).

"Error in setkeyv(x, cols, verbose = verbose, physical = physical) :
Column 'id' is type 'list' which is not supported as a key column type, currently."

average_spc() should therefore receive an enhancement to return a more informative error with regard to the argument supplied to the user.