Add example of changing equations

[paulobruno]

It is possible to do this without changing the package code, but it's not trivial. Currently, we have to look for the equation we want to change in the vector of system equations manually.

[natema]

I think this is fair enough atm. Also, after a first example where we change an equation of the whole system, we can have another example where we change an equation of a subsystem. From that point of view, equations are already grouped in subsystems, so looking for them shouldn't be terrible.

[paulobruno]

I added an example in the Agriculture fig. 4.87.
It is possible to do, but I am not yet comfortable in having to look the number of the equation before changing it.

[universmile]

At the moment, one has to print the array of equations and look at its index, which seems cumbersome. We should find some remedy before implementing the remaining 11 figures that require editing equations, then have it documented.

[natema]

I don't see a natural way to address this point.
Can be an enhancement for a future version, but atm unless you have a proposal, I'll let it as is.

[paulobruno]

Since we have a way to do so, which was the goal of the issue, i'm closing it.
I'll open a new one to improve the current process of changing equations.