add density2d geometry

[bjarthur]

closes #955

[codecov-io]

Codecov Report

Merging #959 into master will decrease coverage by -0.17%.
The diff coverage is 20%.

@@            Coverage Diff            @@
##           master    #959      +/-   ##
=========================================
- Coverage   65.47%   65.3%   -0.17%     
=========================================
  Files          35      35              
  Lines        4814    4828      +14     
=========================================
+ Hits         3152    3153       +1     
- Misses       1662    1675      +13

Impacted Files	Coverage Δ
src/geom/line.jl	91.3% <ø> (-1.01%)	❌
src/statistics.jl	89.57% <21.42%> (-1.22%)	❌
src/guide.jl	84.09% <ø> (ø)	✅

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[tlnagy]

Awesome! I just merged #958, can you rebase this? I'll go ahead and merge once the tests pass.

[tlnagy]

Rebased and merging.