Add support for LSD& Tukey multiple comparisons

[lxw748]

In ecology study, we often apply LSD or Tukey multiple comparisons, which are vital when labeling a batch of significance of experimental result. Most of multiple comparisons methods have been integrated in statmodels.stats.multicomp. You may get more information of multiple comparisons methods from https://zhuanlan.zhihu.com/p/44880434.

[sepro]

We were discussing this before ( https://github.com/trevismd/statannotations/issues/39 ) and it isn't as easy as one might think to support these without a significant re-write. You can however compute these before and use this package to plot the results. I've put up a blog post outlining how to do this.

[rbpisupati]

I have added in a function (https://github.com/rbpisupati/statannotations/commit/53d40eed7efbeef8af66bfa1c59a5ad5dd827fc3). Idea is to add in the letter representation for Tukey's test. The function is very primitive, one need to fix/add it in main function.