Properly measure iteration time in TimePlotter

[jorgepiloto]

💻 Feature request: properly time solvers' iterations

After #13 was merged, I did not notice that the time being measured was the whole required by the algorithm to solve the problem. This is not the time per iteration, as also the initial guess and velocity construction vector are also being considered.

Therefore, we must properly return the time per iteration by starting the clock before the iteration begins and stop it once the solution for the independent variable has been found.

It is no possible to do this without modifying solver behavior, so my plan is to add the tpivariable, which means "time per iteration" as an optional return parameter.

[jorgepiloto]

Because a reduction in the measured time is expected (neither initial guess nor velocity vectors assembly considered), we will probably have to update the scale. I think a range between 0 and 500 microseconds is feasible 👍🏽

[jorgepiloto]

Finally solved! However, a new issue appear while working on this, see #16.