As you can't directly enter NaN/Inf in Excel, there's
no great way to verify behavior. This just ensures
that setting NaN/Inf from Go will result in a #NUM!
error in Excel.
This adds support for extracting a cell's formatted value according to
the number format applied to the cell. To do this we need to implement
a parser for Excel style format strings and support formatting numbers
according to that style.
This also enhances the General formatting to be much closer to what
Excel normally does. There are likely still a few corner cases where
Excel and gooxml differ, but hopefully not too many.
- add a Cell method to the sheet that gets/creates a cell
- chance Row.Cell to not require the row number
- rename EnsureRow to Row
- add examples/tests
