Resolve #194

[dglmoore]

Description

Networks were losing lone nodes when convert the network networkx.DiGraph. This was fixed by explicitly adding all nodes to the network before adding edges. The reason this was necessary is the networkx.DiGraph initializer only adds endpoint nodes when you construct it with a list of edges. Since lone nodes have no edges, they were not included.

Fixes #194

Type of change

• [X] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Documentation

How Has This Been Tested?

Please describe the tests that you ran to verify your changes.

• [x] Create a LogicNetwork with a single node and no edges. The resulting DiGraph has 1 node
• [x] Create a LogicNetwork with two connected nodes and one lone node. The resulting DiGraph as 3 nodes

We have not tested this for WTNetwork on account of #193.

Checklist:

• [X] My code follows the style guidelines of this project
• [X] I have performed a self-review of my own code
• [X] I have commented my code, particularly in hard-to-understand areas
• [X] I have made corresponding changes to the documentation
• [X] My changes generate no new warnings
• [X] I have added tests that prove my fix is effective or that my feature works
• [X] New and existing unit tests pass locally with my changes
• [X] I have added appropriate labels to this pull request
• [X] This is a notable change, and I've added it to CHANGELOG.md

[codecov-io]

Codecov Report

Merging #195 into master will increase coverage by <.01%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #195      +/-   ##
==========================================
+ Coverage   99.13%   99.14%   +<.01%     
==========================================
  Files          16       16              
  Lines        1393     1397       +4     
==========================================
+ Hits         1381     1385       +4     
  Misses         12       12

Impacted Files	Coverage Δ
neet/network.py	93.89% <100%> (+0.19%)	:arrow_up:

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