Add a SequentialExecutor based on ProcessPoolExecutor

[basnijholt]

test with e.g.,

class SequentialExecutor(ProcessPoolExecutor):
    """A asynchronous executor that runs tasks sequentially."""

    def __init__(self) -> None:
        super().__init__(max_workers=1)

def burn():
    import numpy as np
    np.linalg.eigvals(np.random.rand(1000, 1000))

def f(x):
    with ProcessPoolExecutor() as executor:
        future = executor.submit(burn)
        future.result()
    return x**2

learner = adaptive.Learner1D(f, bounds=(-1, 1))
runner = adaptive.Runner(learner, goal=lambda l: l.loss() < 0.1, executor=SequentialExecutor())
runner.live_info()