JSON serialization / deserialization functions for Nodes and Edges

[jacksonwb]

• Add dict (de)serialization helper functions to Node/edge classes
• Add np array (de)serialization methods to np backend
• Update backend tests
• Add (de)serialization tests
• Format changes (sneaky yapf bomb)

[jacksonwb]

@Thenerdstation I've highlighted all the non-formatting changes

[codecov-commenter]

Codecov Report

Merging #824 into master will decrease coverage by 0.02%. The diff coverage is 96.51%.

@@            Coverage Diff             @@
##           master     #824      +/-   ##
==========================================
- Coverage   98.41%   98.39%   -0.03%     
==========================================
  Files         128      129       +1     
  Lines       21639    21856     +217     
==========================================
+ Hits        21297    21506     +209     
- Misses        342      350       +8     

Impacted Files	Coverage Δ
...ensornetwork/tests/network_components_free_test.py	99.36% <66.66%> (-0.18%)	:arrow_down:
tensornetwork/network_components.py	95.64% <89.13%> (-0.20%)	:arrow_down:
tensornetwork/backends/abstract_backend.py	93.91% <92.00%> (-1.59%)	:arrow_down:
tensornetwork/network_operations.py	98.50% <98.73%> (-0.02%)	:arrow_down:
tensornetwork/__init__.py	100.00% <100.00%> (ø)
tensornetwork/backends/numpy/numpy_backend.py	98.52% <100.00%> (+0.06%)	:arrow_up:
tensornetwork/backends/numpy/numpy_backend_test.py	99.28% <100.00%> (+<0.01%)	:arrow_up:
tensornetwork/tests/serialize_test.py	100.00% <100.00%> (ø)

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