Simultaneous CI option for iplot, coefplot

[s3alfisc]

1. Adds a joint method to plot simultaneous CIs via the iplot() and coefplot() functions & methods (only for single methods).
2. Renames the CI columns in confint() output to per cent, i.e. "0.025" becomes "2.5" (to standardize with the output of tidy).
3. Updates the did vignette.
4. Lots of linting from past PRs + etable now returns None at the end to accommodate mypy.
5. Working for mlp and lets_plot versions.

Relates to #586, fyi @apoorvalal

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[s3alfisc]

[apoorvalal]

Very nice! I was thinking about contributing this too.
I computed coverage rates and find that only the joint method has nominal coverage here. Worth adding to the vignette as motivation for joint intervals?

https://gist.github.com/apoorvalal/8a7687d3620577fd5214f1d43fc740b3

[s3alfisc]

Yes, this would be very cool to have in a vignette! Would you add it to the did/event study notebook or should we add a dedicated notebook on simultaneous inference correction where we show you simulation + also illustrate the rwolf & bonfetroni functions?