Add more swarm-based metaheuristics

[HeleNoir]

This is a part of #75 and includes PSO-like algorithms:

• Gravitational Search (e.g. https://linkinghub.elsevier.com/retrieve/pii/S2210650217303577)
• Firefly Algorithm (e.g. http://link.springer.com/10.1007/978-3-642-04944-6_14)
• Bat-inspired Algorithm (e.g. http://link.springer.com/10.1007/978-3-642-12538-6_6)
• Black Hole Optimization (e.g. http://link.springer.com/10.1007/978-3-319-11017-2_7)
• Dragonfly Algorithm (e.g. http://link.springer.com/10.1007/s00521-015-1920-1)
• Grey Wolf Optimizer (e.g. https://linkinghub.elsevier.com/retrieve/pii/S0965997813001853)

As the algorithms are very similar, I would implement them at the same time, i.e. in the same PR.