Midpoint - working but slow, architecture change needed

The midpoint works if computed for any given value.
But everytime, every single previous point is recomputed, which is a
huge waste.

Instead, an IntegralCurve should be created, that basically does the
same as the FunctionCurve but compute all value.
Alternatively, AbstractFunction could be modified to have a
computeRange(min, max, resolution) function, which would compute all the
data as once.
By redifining it for the IntegralFunction, it would allow to compute it
efficiently.
Integral function/Curve working. Interface not done

Computing integrals through the IntegralFunction class works.
IntegralCurves displays them.

Now the interface has to be done to control them.
Configure integralcurve stepnumbers
Compute functions with parameters - ex: f(V+e-3*a)
Function manages its own parameters
Integrate using function and parameters (1/2)

XXX: hack had to be used
The compiler wouldn't let me use Function in IntegralFunction.cpp
because of some very weird circular inclusion that I couldn't find
Fix that
Fix displaying of integral curve (to modify)
Integrate product of functions. Parameters to handle now
(Just an intermediate save)
Integrate using function and parameters (1/2) + optimize midpoint
Draft of Integral creation interface (looks like it's working)
Use Function where appropriate
Cleanup
Improve integral config dialog
Fix Hierarchical function update
Fix modification of variables for integral curves
Update Integralcurve on variable change
Fix attachement to plot
Fix loading of HierarchicalFunctions
Save/load integrals. Now move parameters outside of function!!