Add copy_spins!. Expose jitter argument in minimize_energy!

[kbarros]

1. Expose the jitter argument to minimize_energy! in the docs.
2. Introduce a copy_spins! function.
3. Update the minimize_energy! doc string. It now suggests this snippet to search for the ground state:

sys_min = clone_system(sys)
for i in 1:100
    randomize_spins!(sys)
    minimize_energy!(sys)
    if energy(sys) < energy(sys_min)
        copy_spins!(sys_min, sys)
    end
end
copy_spins!(sys, sys_min)

[ddahlbom]

This looks good to me. It will be nice to have this parameter exposed.