add possibility to specify a list of spreadsheets/excels/.. so that all the sheets in the files are considered together, concatenating the content_index sheets if they are defined in multiple files (names of sheets must be unique and it should not matter which file a sheet belongs to)
add possibility to specify a list of spreadsheets/excels/.. so that all the sheets in the files are considered together, concatenating the content_index sheets if they are defined in multiple files (names of sheets must be unique and it should not matter which file a sheet belongs to)