ENH: Adds id to support output caching

[thomasjpfan]

Fixes #39

This PR adds an optional id field to data dictionary. When cache_output is set to True, theid field is appended to step.nameto distinguish between output caches produced by different data dictionaries.

For example:

data_train = {
    'id': 'data_train'
    'input': {
        'features': np.array([
            [1, 6],
            [2, 5],
            [3, 4]
        ]),
        'labels': np.array([2, 5, 3]),
    }
}
step = Step(
    name='test_cache_output_with_key',
    transformer=IdentityOperation(),
    input_data=['input'],
    experiment_directory='/exp_dir',
    cache_output=True
)
step.fit_transform(data_train)

This will produce a output cache file at /exp_dir/cache/test_cache_output_with_key__data_train.

[jakubczakon]

@thomasjpfan Sorry for late answer.

It's a very interesting idea from the production point of view where your training/dev/test data can easily change. Having an Id here could save you time and trouble.

We're gonna think it through shortly and get back to you.

[kamil-kaczmarek]

@thomasjpfan As mentioned in issue I will take a closer look at it next week. Thank you for your PR, here!