Correct CGP behaviour regarding verbosity and Restorable API

[nhuet]

This PR fixes #177.

• We remove all prints when verbose=False
• We remove inheritance from Restorable since CGPWrapper did not implement _save() nor _load().

To see how verbosity is fixed you can compare how the following snippets behave before and after the PR:

import gym
from skdecide.hub.domain.gym import (
    GymDomain,

)
from skdecide.hub.solver.cgp import CGP


ENV_NAME = "MountainCarContinuous-v0"
domain_factory = lambda: GymDomain(gym.make(ENV_NAME))

print("*** With verbosity")
solver = CGP("TEMP_CGP", n_it=2, verbose=True)
GymDomain.solve_with(solver, domain_factory)

print("***Without verbosity")
solver = CGP("TEMP_CGP", n_it=2, verbose=False)
GymDomain.solve_with(solver, domain_factory)

[neo-alex]

OK for me, thanks