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Formulas wrongly returned 0 instead of NULL

This was because PHPExcel returned FALSE in som edge cases
and it was later cast to 0
Revert commit 170d049, do not sum children if answer exists

We must not sum children if an answer exists for a filter. The presence
of an answer override everything that may or may not exists in children.

What commit 170d049 tried to fix must actually be fixed by modifying our
filter tree structure and/or entering answers on the right filters. So
basically fix the original data, not our computing algorithm.
Revert commit 74c9aef, because it was a WIP which was never finished nor needed
Formula importation is aware of filter tree remplacements

It previously refered to newly injected filter, instead of the original
one, making al formula evaluation fail during computation, since
it was the not the right one.