Support for Simultaneous CIs in coefplot() and iplot()

[s3alfisc]

Context

The confint() method allows for simultaneous confidence intervals via a multiplier bootstrap when setting joint = True. It would be nice to be able to be able to plot simultaneous confidence intervals.

To Do

• Store an object self._conf_int_simult in the Feols / DiD classes that is a empty np.array by default
• Add an argument joint to the following methods: Feols.coefplot, Feols.iplot, TWFE.iplot, DID2S.iplot.
• If joint = True, use simultaneous CIs for plotting the confidence intervals.
• If simultaneous CIs are not computed, run confint(..., joint = True) and assign the result to self._conf_int_simult
• Currently, I do not think that we can compute joint CIs for the LPDID.iplot, we'd have to compute a vcovSUR variance covariance.matrix of the stacked regressions Inference: Implement vcovSUR for seemingly unrelated regression #428.
• Add some basic API tests.