Add support for dependent tasks

[jan-janssen]

from time import sleep
from pympipool import Executor

def add_function(parameter_1, parameter_2):
    sleep(1)
    return parameter_1 + parameter_2

with Executor(max_cores=1, backend="mpi") as exe:
    future_1 = exe.submit(add_function, 1, parameter_2=2)
    future_2 = exe.submit(add_function, 1, parameter_2=future_1)
    print(future_2.result())