Formula support

[elibyy]

Hey there,
I always needed the formula in my reports, so I've made an implementation of that, And it would be nice to have it within the original package.

P.S. Really needed that on production ASAP so had to change the composer package, if this causes a problem with the PR, I can fork to different repo

Thanks
Eli

[barryvdh]

This shouldn't be changed in your PR

[elibyy]

Yes, as being said I needed this in prod, so I'll commit another with the correct package name

[barryvdh]

So basically, this sets the formula without calculation it, right? So it wouldn't affect performance (much)?

Looks nice. Similar to #425 but this allows to set the pre-calculated value, which is nice :)

[elibyy]

This ia based on that PR but with two changes

1. There's a class and not just guessing by equals sign
2. You can set the pre calculated value so if the user opens it in excel without enabling editing they would still have a value

[barryvdh]

Should this be targeted at the v3 branch? As that is coming soon(ish) as it seems?

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[Slamdunk]

Merged a variation in openspout/openspout#5