Implement OptimizerProtocol for mypy checking.

[c-bata]

To implement CMA-ES variants, it is better to provide a protocol for optimizer API (See PEP 544 or Python docs for details).

Probably, the protocol will be like this:

import numpy as np
from typing import List, Optional, Tuple

try:
    from typing import Protocol
except ImportError:
    from typing_extensions import Protocol


class Optimizer(Protocol):
    @property
    def dim(self) -> int:
        """A number of dimensions"""

    @property
    def generation(self) -> int:
        """Generation number which is monotonically incremented
        when multi-variate gaussian distribution is updated."""

    @property
    def population_size(self) -> int:
        """A population size"""

    def ask(self) -> np.ndarray:
        """Sample a parameter"""

    def tell(self, solutions: List[Tuple[np.ndarray, float]]) -> None:
        """Tell evaluation values"""

    def set_bounds(self, bounds: Optional[np.ndarray]) -> None:
        """Update boundary constraints"""