Support lambda functions as objective functions

[sffc]

The objective function is required to be fn(&Particle, usize, &Vec<usize>) -> f64, but it would be more flexible if it would be impl Fn, so that my objective function can read from local variables.

[czonios]

That's a good idea. Would you be willing to submit a PR with this change?

[sffc]

I would, but I found a workaround.

[ai-mannamalai]

it works; see my PR #4