improve the bivariate normal pdf 3d plot example

[kindlychung]

Used formula from wolfram mathworld to define the pdf function.

[kindlychung]

[codecov[bot]]

Codecov Report

Merging #197 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #197   +/-   ##
=======================================
  Coverage   68.83%   68.83%           
=======================================
  Files          56       56           
  Lines        4313     4313           
=======================================
  Hits         2969     2969           
  Misses       1344     1344           

Impacted Files	Coverage Δ
src/coord/ranged3d/projection.rs	73.86% <0.00%> (-1.14%)	:arrow_down:
src/coord/ranged1d/types/slice.rs	90.90% <0.00%> (+2.27%)	:arrow_up:

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