Proposed changes

This PR updates the uplift and Qini curve plots with a fixed random line. Currently, we use the average of the ten sets of random estimations. Although it is close to the theoretical straight line, it is not completely the same, and it has caused some confusion (ref: #798, #706). The fixed random should help clarify some of these.

It also simplifies the plot*() functions by removing duplicates between plot_tmlegain() and plot_tmleqini().

Note that get_cumlift(), get_cumgain(), and get_qini() no longer output a column called Random for the random estimate.

Types of changes

What types of changes does your code introduce to CausalML?
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• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

• I have read the CONTRIBUTING doc
• I have signed the CLA
• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged and published in downstream modules

Further comments

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