add a blocking_pbar

[delgadom]

# forward compatibility
try:
    from tqdm.auto import tqdm
except NameError:
    from tqdm import tqdm_notebook as tqdm

# progress bar that blocks until completion
def blocking_pbar(futures, **kwargs):
    pbar = tqdm(dd.as_completed(futures), total=len(futures), **kwargs)
    for _ in pbar:
        errors = [f for f in futures if f.status == 'error']
        if len(errors) > 0:
            pbar.set_postfix({'errors': len(errors)})

[delgadom]

lol. This is the same thing as #53. Focus, @delgadom.