Add benchmark simulation studies

[t-tte]

Proposed changes

This PR partly addresses issue #109 by adding a notebook comparing meta-learners across a number of simulation setups/parameters.

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[paullo0106]

The notebook looks good and helpful to me! I'm curious though, is it expected that gradient boosting with R-learners would perform worse across other meta-learners and these 4 data generation setups?

[t-tte]

That's a great question @paullo0106. I suspect it is possible to improve performance by parameter tuning, but I wanted to keep this exercise computationally manageable. In order to fully replicate the results from the paper, one would need to cycle through the hyperparameters specified therein.