Add minimize key to minimize fitness value, rather than maximize

Hey!

Thanks for the great DE library! It's soo fast!

I often want to minimize my objective functions, and use multiple optimization algorithms on the same objective function.

Thus I made a key to minimize the objective function with PyFDE. A description of the changes:

• Add minimize key to init

• By default minimize=False

• Add check in evaluate()for minimize key, if minimize=True the fitness value stored will be multiplied by -1

• Add assert in init to ensure that minimize is not compatible with batch mode

• Added minimize key description in the tutorial for documentation

Thanks,
CJ

EDIT 2018/05/10

• Batch and minimize now supported

• If statement in evaluate checks for minimize