Date base is taken into account for date formulas

Date base (1900 or 1904) wasn't taken into account in `read-cell` for
formula date cells.
Now it's fixed. `read-cell-value` for `Cell/CELL_TYPE_NUMERIC` doesn't use
`date-format?` internally, but isn't removed to save backward
compatibility. Moreover workbook method that can show if workbook uses 1904 base date (e.g. `XSSFWorkbook.isDate1904`) is protected.
However `read-cell-value` itself doesn't consider date base as it isn't
present in cell value and requires changing method args.

P.S.: test files sizes are smaller probably because thumbnails are not included.