Dev/717 pytest tau col hotfix

Proposed changes

This is work to resolve the issue described in #717.

Types of changes

What types of changes does your code introduce to CausalML? Put an x in the boxes that apply

[X] 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

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

Further comments

A variety of errors arise when currently building as described in #717, this PR resolves those errors. Errors were due to non-existent keys from two sources:

most errors were due to visualization functions like get_cumgain looking for a treatment_effect_col named "tau" by default, but the treatment effect column being passed was called "treatment_effect_col" without supplying the new name as an argument to the function
errors in test_causal_trees.py were due to indexing the wrong element of what is returned by qini_score(...)

Starting this PR to see how it builds on the uber fork and will resolve issues as required.

uber / causalml