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.
Array formulas are entered in Excel by pressing
Ctrl+Shift+Enter to finish the formula. The resulting
formula is entirely in one cell, but its computed results
expand into cells the size of the array result.
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.
