Add support for Vine Copulas as a modeler.

[ManuelAlvarezC]

It could be useful to add support for different models. To archieve that we should:

1. Wait for this issue of Copulas is done and released.

2. Update our requirements to work with the latests version of copulas.

3. Add a new method sdv.Modeler.flatten_dict, that gets a nested dictionary and returns it flattened:

    >>> nested_dict
    {
        'one_attribute': 0
        'nested_attribute': {
            'foo': 'bar
        }
    }
    
    >>> sdv.Modeler.flatten_dict(nested_dict)
   
    {
       'one_attribute': 0
       'nested_attribute__foo': 'bar'
    }

4. Add a new method sdv.Sampler.unflatten_dict that does the exact opposite, that is:

    >>> assert nested_dict == sdv.Sampler.unflatten_dict(sdv.Modeler.flatten_dict(nested_dict))
    >>> assert flattened_dict = sdv.Modeler.flatten_dict(sdv.Sampler.unflatten_dict(flattened_dict))

5. Change the behavior of sdv.Modeler.flatten_model in order for it recieve a modelas input and return a pandas.Series with the flattened model dict.

6. Rename the distribution keyword on sdv.Modeler.__init__ to model_kwargs that defaults to None, but when its present its passed to model when instances are created.

7. Change the behavior of sdv.Sampler._make_model_from_params to make that after the parameters have been retrieved from the parent_row are transform into a dictionary, passed to sdv.Sampler.unflatten_dict and the result to model.from_dict