Unintuitive adapter syntax

[grzes314]

Current syntax for adapters has some peculiarities.
Consider the following example.

        step = Step(
            name='ensembler',
            transformer=Dummy(),
            input_data=['input_1'],
            adapter={'X': [('input_1', 'features')]},
            cache_dirpath='.cache'
        )

This step basically extracts one element of the input. It seems redundant to write brackets and parentheses. Doing
adapter={'X': ('input_1', 'features')},
should be sufficient.

Moreover, to my suprise
adapter={'X': [('input_1', 'features'), ('input_2', 'extra_features')]},
is incorrect, and currently leads to
ValueError: too many values to unpack (expected 2)

My suggestions to make the syntax consistent are:

1. adapter={'X': ('input_1', 'features')} should map X to extracted features.
2. adapter={'X': [...]} should map X to a list of extracted objects (specified by elements of the list). In particular adapter={'X': [('input_1', 'features')]} should map X to a one-element list with extracted features.
3. adapter={'X': ([...], func)} should extract appropriate objects and put them on the list, then func should be called on that list, and X should map to the result of that call.

[jakubczakon]

@grzes314 great idea, it seems intuitive to have it as you described.
Current api is (especially for just one element) very clunky. Also I think one option that could be useful is to be able to pass a chain of functions simply for example np.exp(lambda x: x[0]) or something.

[kamil-kaczmarek]

merged in #42. Closing this.