Update documentation with number format

Fix constant usage in example

Add support for cells in error when writing XLSX and ODS

When appending data to an existing sheet, it was possible to get cells in error when reading (DIV/0 for instance). When trying to write them back, `addRow` would throw because `Cell`s in error were not supported by the writers.

Fix reading of 1904 dates option

Whether the spreadsheet is using 1904 dates or not is controlled by a XML property. Its value can be the string "false" that is not mapped to the boolean "false" but to the boolean "true"... Therefore Spout was previously using the wrong date system when this property was set.

Merge branch 'master' into develop_3.0

Change Worksheet function visibility from private to protected in ord…

…er to class extending. We have created small package on top of the Spout for support cell number format and per cell styles.

Prevent error when close() called while writer already closed (#402)

Fix crash when using associative array with empty row (#354)

Move ReaderCommonOptions class to Common folder (#343)

Apply custom style to empty cells if needed (#307)

Fixes #295

If a row should be written with a custom style, the handling of empty cells should change.
Instead of being skipped entirely, empty cells will be applied the custom style, if this style has custom background color or borders.
If not, then the cell definition can still be skipped.