Decorator for cost functions

[whzup]

Is your feature request related to a problem? Please describe.
Until now the cost function had to return the shape (n_particles, ) which has caused some problems in using the package.

Describe the solution you'd like
I propose a decorator so we can just mark the cost function:

@pyswarms.cost
def cost_function(x, **kwargs):
    cost = do_something_costly(x, **kwargs)
    return cost

This lets the user write a "strategy" that has to be applied to all the particles in the swarm in order to compute the cost.

Describe alternatives you've considered
N/A

Additional context
The hypothesis module has something similar where users can create custom strategies for the testing by using a decorator