Add formula functionality in v3.3.0

[davidbonting]

Since this package doesn't support formula's.

5 years ago there was a try #425 and 3 years ago too #612. Sadly they were never merged and the package has changed so the code doesn't work anymore.

I liked the idea of #425 more then #612, so I just used that one as reference. If anyone wants to use my code (since this PR will probably never be merged) here's what you got to do:

{ 
...
 "repositories": [
        {
            "type": "git",
            "url": "https://github.com/davidbonting/spout.git"
        }
    ],
 "require": {
      "box/spout": "master"
      ...
 }
}

And in the code:

 // This will show empty if in preview mode
$rowFromValues = WriterEntityFactory::createRowFromArray(['Total', '=SUM(E2:E3)']);

// This will show a cached value (120) if in preview mode
$rowFromValues = WriterEntityFactory::createRowFromArray(['Total', ['=SUM(E2:E3)', 120]]);

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[Slamdunk]

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