Formula generation: reduce number of initial terms

[ckrause]

In getNumInitialTermsNeeded() we use the number of stateful cells to estimate the number of needed initial terms. This is actually an overcount. We could reduce it by computing a dependency graph of the stateful cells and use the length of its longest path instead.

https://github.com/loda-lang/loda-cpp/blob/main/src/form/formula_gen.cpp#L194