Write formulas to XLSX files.

[stevesweets]

Ref this issue: https://github.com/box/spout/issues/424

I utterly understand that reading formulas is a pain. It should be noted that writing formulas to ODS is rather a pain, too, but the nature of xlsx actually makes it rather easy to write them.

This way, we can use the super-performant spout when we want to generate an xlsx file that needs to contain some excel formulas, and I don't think this lands outside of the goals of Spout.

I await your judgement!

[boxcla]

Verified that @stevesweets has signed the CLA. Thanks for the pull request!

[stevesweets]

This works for this, too:

https://github.com/box/spout/pull/352

[adrilo]

Thanks @stevesweets ! Most likely, this won't be merged until v3 is released. v3 should make it easier to work with formulas. But I'll keep the PR open, as a reference for future implementation.

[CLAassistant]

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[StefanoGroenland]

Will this PR ever get merged?

[Slamdunk]

Merged a variation in https://github.com/openspout/openspout/pull/5