Added basic support of Line, LineSegment and LineLoop

[danieljfarrell]

The changes were trivial in the end. I just added the following,

class Line(Object):
    _type = u"Line"

class LineSegments(Object):
    _type = u"LineSegments"

class LineLoop(Object):
    _type = u"LineLoop"

I spend most of the time testing the different rending options (Line, LineSegement and LineLoop) in the included lines.ipynb. In this notebook I also played around a little with using materials to give the lines different colours. It seems they can be rendered using standard mesh materials.

I also thought about adding https://threejs.org/docs/#api/en/materials/LineBasicMaterial and https://threejs.org/docs/#api/en/materials/LineDashMaterial but decided against it so as to keep this pull request simple.

[codecov-io]

Codecov Report

Merging #43 into master will decrease coverage by 13.22%. The diff coverage is 94.23%.

@@             Coverage Diff             @@
##           master      #43       +/-   ##
===========================================
- Coverage   87.35%   74.13%   -13.23%     
===========================================
  Files           8        8               
  Lines         609      661       +52     
===========================================
- Hits          532      490       -42     
- Misses         77      171       +94

Impacted Files	Coverage Δ
src/meshcat/geometry.py	62.23% <92.3%> (-24.73%)	:arrow_down:
src/meshcat/tests/test_drawing.py	71.31% <96.15%> (-24.8%)	:arrow_down:

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