Add composite LRP example

[adrhill]

An example on how to use composite LRP is missing from the docs and frequently requested.

Closes #252
Addresses #162, #190, #249, #255

---

This PR adds a Jupyter notebook explaining how to use the keyword arguments of the LRP analyzer class.
I'm opening this as a draft PR as we should

• add references
• check whether the example in the notebook can be improved

The references I would add are

• Lapuschkin et al.: Understanding and Comparing Deep Neural Networks for Age and Gender Classification
• Montavon et al.: Layer-Wise Relevance Propagation: An Overview

Maybe @sebastian-lapuschkin you can help me out here?

[adrhill]

Here's a screenshot of the example in the notebook for quick reference:

[codecov-commenter]

Welcome to Codecov 🎉

Once you merge this PR into your default branch, you're all set! Codecov will compare coverage reports and display results in all future pull requests.

Thanks for integrating Codecov - We've got you covered ☂️

[sebastian-lapuschkin]

another reference to add would be

Kohlbrenner et al.: Towards Best Practice in Explaining Neural Network Decisions with LRP

which properly introduces the composite LRP pretty much at the same time as Montavon et al (preprints existed a bit earlier) and evaluates the composite approach against other methods and approaches.

is there anything else I can help with?

[adrhill]

is there anything else I can help with?

Is there anything you would change about the example in the screenshot?

[sebastian-lapuschkin]

maybe add some line(s) actually using this custom built analyzer, and some example heatmaps of this composite rule with non-composite counter parts for comparison?

a hint at the existing LRP presets (if they will be carried over to version 2.0) would also be interesting to the users I guess.

[adrhill]

A usage example is already in the notebook.
I will add a comparison with the existing presets.

Thanks!