Implement Parallel and Distributed GP

[bgalvao]

As a baseline, implement island-model

[bgalvao]

Island model is implemented by default. Here's an idea:

• include a field in Mgp called migration_graph so as to define migrational directions

Only a possible feature, as I do not plan on complicating the implementation of Meta-GA nor on increasing its complexity.

[bgalvao]

Here is a must-do:

• determine the number of migrants.

which should be implemented by changing Mgp.core to a tuple (GP, u16)