Add python bindings for pointcloud and contact retrieval

[chhinze]

1. Adds Python bindings for the following:

   • Extracting contacts from CollisionResult object (Methods getContact and getContacts)
   • PointCloudShape, so it is now possible to draw pointclouds in dartpy as well

2. Adds an example python/examples/contacts_pointcloud/main.py, which is basically the hello_world_gui example (robot and ground are set transparent) with detection of contacts and visualization as a pointcloud before each GUI refresh.

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• Document new methods and classes
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[codecov]

Codecov Report

Merging #1455 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1455      +/-   ##
==========================================
+ Coverage   58.21%   58.23%   +0.02%     
==========================================
  Files         412      412              
  Lines       29923    29923              
==========================================
+ Hits        17419    17425       +6     
+ Misses      12504    12498       -6     

Impacted Files	Coverage Δ
dart/dynamics/EulerJoint.cpp	73.00% <0.00%> (+3.68%)	⬆️

[jslee02]

Looks good to me. Thanks for adding these bindings.

[chhinze]

Thank you for reviewing and merging.