[REVIEW] enable multigraph

[Iroy30]

No description provided.

[GPUtester]

Please update the changelog in order to start CI tests.

View the gpuCI docs here.

[codecov-io]

Codecov Report

Merging #1280 (b6fa160) into branch-0.17 (6b92349) will increase coverage by 0.32%.
The diff coverage is 91.66%.

@@               Coverage Diff               @@
##           branch-0.17    #1280      +/-   ##
===============================================
+ Coverage        60.38%   60.70%   +0.32%     
===============================================
  Files               67       67              
  Lines             3029     3031       +2     
===============================================
+ Hits              1829     1840      +11     
+ Misses            1200     1191       -9     

Impacted Files	Coverage Δ
python/cugraph/__init__.py	100.00% <ø> (ø)
python/cugraph/structure/graph.py	68.33% <80.00%> (+1.54%)	⬆️
python/cugraph/structure/__init__.py	100.00% <100.00%> (ø)
python/cugraph/structure/symmetrize.py	73.33% <100.00%> (+2.60%)	⬆️
python/cugraph/traversal/sssp.py	93.63% <100.00%> (ø)
python/cugraph/utilities/utils.py	71.79% <100.00%> (+0.24%)	⬆️

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[afender]

I think it is best to capture our multigraph discussions in the form of multigraph class documentation and add it as part of this PR from the start while it is still fresh in our mind.

[rlratzel]

Looks good, just a few minor requests.