Formula processor

[RedNeath]

The role of the processor will be to use the calculation tree to compute every node, one after the other, using a DFS algorithm.

Here's what is will have to do on each node:

1. Pre-process the node; call the pre-processor to make sure there are no incoherence and the calculation can be accomplished.
2. Calculate the node's value.
3. Remove the leaves (children of the current node) from the memory
4. Replace the node's content with the result of the calculation.