Make the optimizer available using fpm

[fedebenelli]

Hi! I've seen your code and it can be really useful in some of my works. I've did some editions to it to make it able to use it as a dependency with the Fortran Package Manager fpm, this will be helpful for me and other users that want to use your code in their work.

Besides that compatibility, I did some minor changes to make it easier to use in other projects. Now the optimizer routine is included in a module instead of being a main program, and receives the desired fitness function as an argument. An usage example can be seen at the new directory example:

program main
    use fitness_example, only: fitness
    use PSOclassicG, only: pso
    use iso_fortran_env, only: pr => real64

    real(pr) :: x(30), xmin(30), xmax(30)

    xmax = 30
    xmin = -30
    call pso(fitness, x, psize=30, verbose=.true., xmin=xmin, xmax=xmax)
end program main

With fpm installed an user can try this example by running fpm run --example inside the repo folder.

Other way of using it is making a fpm project and including this package as a dependency in the fpm.toml manifest file

[dependencies]
Particle-Swarm-Optimization-Fortran-95 = {git =https://github.com/zaman13/Particle-Swarm-Optimization-Fortran-95 }

Without downloading anything manually now the optimizer can be available to call in another project.

I don't know much details of the algorithm itself, but this could be further improved to have a nice package for other users :)

[fedebenelli]

More work can be done, but I avoided touching the code as much as I could since I'm not an expert of the algorithm itself.