Enforce formula node order and properly convert cell type to number

[afalkenhahn]

For some reason my Excel version (Excel 2016) refuses to load files where <v> comes before <f>. For example, the following XML fails to load in Excel 2016:

<v>0</v><f>A1+A2</f>

Whereas this works:

<f>A1+A2</f><v>0</v>

I didn't find this documented anywhere but anyway, this pull request moves the <f> node to the front so that the formula always comes first.

It also fixes two issues in setFormulaString, namely that setFormulaString set the cell value to 0 but didn't remove the t attribute so that it could still be set to something like inlineStr. Also, setFormulaString didn't remove any potential <is> node containing an inlined string which could also break XLSX documents because it could happen that there is both, an <is> and a <v> node inside the cell value definition because setFormulaString will add a <v> node without clearing an <is> node which might already be there.