Implement continuous version

[artur-deluca]

The package started as a particle swarm optimizer for discrete optimization. However, this doesn't mean that it cannot be extended for the original continuous spectrum without too much of a hassle.

[artur-deluca]

One cool example would be the original testing from Kennedy and Eberhart also discussed on Van der Behrgs' thesis (pg. 198)

[artur-deluca]

Additionally, this could bring a different arrangement to the package. Instead of having:

from psopt.permutation import Permutation
from psopt.combination import Combination

which is kind of useless today, we could have:

from psopt.discrete import Permutation
from psopt.discrete import Combination
from psopt.continuous import Continuous # or some other name