Enhancements and fixes to PairGrid

[mwaskom]

A number of small enhancements are combined here:

• Added the corner option to :class:PairGrid and :func:pairplot to make a grid with only the lower triangle of bivariate axes. (I don't believe there is an open issue about this but there's a popular SO question.)

iris = sns.load_dataset("iris")
g = sns.pairplot(iris, height=1.5, corner=True)

• Generalized the idea of "diagonal" axes in :class:PairGrid to any axes that share an x and y variable. (Closes pairplot diag_kind does not work on non-square plots #1125)

iris = sns.load_dataset("iris").drop("species", axis=1)
g = sns.PairGrid(iris, height=1.5, x_vars=iris.columns, y_vars=iris.columns[:3])
g.map_diag(sns.kdeplot)
g.map_offdiag(sns.scatterplot)

• In :class:PairGrid, the hue variable is now excluded from the default list of variables that make up the rows and columns of the grid. (Supersedes and closes remove hue from pairplot variables (fix #1502) #1525

• Exposed the layout_pad parameter in :class:PairGrid and set a smaller default than what matptlotlib sets for more efficient use of space in dense grids.

• Fixed the behavior of dropna in :class:PairGrid to properly exclude null datapoints from each plot when set to True. This is relatively less necessary these days as matplotlib is generally better about handling nans, but still potentially useful. This also prompted a helpful refactor of the PairGrid code. Closes PairGrid and missing values?  #407, closes ENH: Pairwise dropna in PairGrid. Closes #407. #409 (!), and supersedes and closes Implemented dropna bug fix for pairplot #1718.

[codecov]

Codecov Report

Merging #1824 into v0.9 will decrease coverage by 0.01%.
The diff coverage is 93.33%.

@@            Coverage Diff             @@
##             v0.9    #1824      +/-   ##
==========================================
- Coverage   94.19%   94.18%   -0.02%     
==========================================
  Files          15       15              
  Lines        4532     4538       +6     
==========================================
+ Hits         4269     4274       +5     
- Misses        263      264       +1

Impacted Files	Coverage Δ
seaborn/axisgrid.py	94.65% <93.33%> (-0.09%)	⬇️

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[codecov]

Codecov Report

Merging #1824 into v0.9 will increase coverage by 0.03%.
The diff coverage is 97.7%.

@@            Coverage Diff             @@
##             v0.9    #1824      +/-   ##
==========================================
+ Coverage   94.19%   94.23%   +0.03%     
==========================================
  Files          15       15              
  Lines        4532     4541       +9     
==========================================
+ Hits         4269     4279      +10     
+ Misses        263      262       -1

Impacted Files	Coverage Δ
seaborn/axisgrid.py	94.93% <97.7%> (+0.18%)	⬆️

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