[PR] add args parameter for GA class

[qelloman]

Related Issue

#71

Description

• Previously, fitness_func takes only two arguments: solution and solution index. If extra parameters are required for the fitness_func, we are forced to use global parameters as shown in 'example.py', which is inconvenient.
• Like scipy.optimize.differential_evolution, a new parameter, args, is added to GA class.
• Tuple args is passed to fitness_func so it can use fixed parameters. => fitness_func(solution, solution_idx, args)