Stat.smooth() confidence bands

[Mattriks]

• I've updated the documentation to reflect these changes
• I've added an entry to NEWS.md
• I've added and/or updated the unit tests
• I've run the regression tests
• I've built the docs and confirmed these changes don't cause new errors

This PR:

• Adds confidence bands for Stat.smooth(method=:lm)
  (but not for method=:loess, because currently Loess.jl doesn't return the loess standard error Is there a way to see the shaded confidence region in Geom.smooth #519)
• Updates Geom.ribbon (fixes Order dependent incorrect handling of NaN in Geom.ribbon #1167, closes Geom.ribbon in does not accept Int vectors as input when used with Geom.smooth #559)
• Adds gallery examples for Stat.smooth()

Examples

salaries = dataset("car","Salaries")
salaries.Salary /= 1000.0
salaries.Discipline = ["Discipline $(x)" for x in salaries.Discipline] 

p = plot(salaries[salaries.Rank.=="Prof",:], x=:YrsService, y=:Salary, 
    color=:Sex, xgroup = :Discipline,
    Geom.subplot_grid(Geom.point,
     layer(Stat.smooth(method=:lm, levels=[0.95, 0.99]), Geom.line, Geom.ribbon)), 
    Scale.xgroup(levels=["Discipline A", "Discipline B"]),
    Guide.colorkey(title="", pos=[0.43w, -0.4h]), 
    Theme(point_size=2pt,
        lowlight_color=c->RGBA{Float32}(c.r, c.g, c.b, 0.2) )
)

x = range(0.1, stop=4.9, length=30)
D = DataFrame(x=x, y=x.+randn(length(x)))

p = plot(D, x=:x, y=:y, Geom.point,
     layer(Stat.smooth(method=:lm, levels=[0.90,0.99]), Geom.line, Geom.ribbon(fill=false)),
     Theme(lowlight_color=c->"gray", line_style=[:solid, :dot])
)

[codecov-io]

Codecov Report

Merging #1231 into master will increase coverage by 1.57%.
The diff coverage is 95.65%.

@@            Coverage Diff            @@
##           master   #1231      +/-   ##
=========================================
+ Coverage   83.53%   85.1%   +1.57%     
=========================================
  Files          35      35              
  Lines        4033    4103      +70     
=========================================
+ Hits         3369    3492     +123     
+ Misses        664     611      -53

Impacted Files	Coverage Δ
src/geom/ribbon.jl	100% <100%> (ø)	⬆️
src/misc.jl	62.82% <100%> (+12.82%)	⬆️
src/statistics.jl	89.45% <92.68%> (+1.79%)	⬆️
src/varset.jl	97.77% <0%> (-2.23%)	⬇️
src/Gadfly.jl	75.3% <0%> (-0.43%)	⬇️
src/guide.jl	83.65% <0%> (+0.28%)	⬆️
src/shapes.jl	98.03% <0%> (+0.98%)	⬆️
src/theme.jl	70.17% <0%> (+3.5%)	⬆️
src/geometry.jl	75% <0%> (+3.57%)	⬆️
... and 2 more

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[Mattriks]

Added a unit test (in testscripts/nan_skipping.jl), rebased and squashed commits - done!