Added methods to output heuristic models results

[iago]

Methods added to get outputs with different granularity individually for all heuristic models. attrib.<HEURISTIC_MODEL>_journeys() attrib.<HEURISTIC_MODEL>_channels()

Example: attrib.attribution_linear()[1] was used to access the results of Linear model with channel granularity. Now, it's possible to access them calling attrib.linear_channels()

If the method that calculates the results is not called before the new methods a warning is issued.

Next steps:

• Call the method that calculates the outputs instead of issuing a warning if it hasn't been run yet;
• Create similar methods for algorithmic models. So it will be possible to access Markov transition matrix directly, for example.

[rsennes]

Thanks Iago,

We took some time to evaluate it but now it's already implemented in the code. Thanks for the contribution.