Move finalsize specific element from solver function to main finalsize() function

It would be good to refactor the code to extract the parts specific to finalsize from the solver functions. In other words, the solver function should only contain the solver algorithm and nothing more.

- it will avoid redundant code across solvers:

https://github.com/epiverse-trace/finalsize/blob/185e99f1f14f1d885f78825540d6dd02e73736b4/R/iterative_solver.R#L30-L48

- it will make it easier to plug external solvers, such as solvers provided by base R (e.g`optim()`) or other packages.