Add test for nodes reducer

[nolleto]

• Create tests for src/reducers/nodes.js. PR reference: Add test for arrows reducer #39
• Refactor src/reducers/nodes.js.

To test it properly you will need a network so, you can use these networks: networks.zip
You can create a folder __fixtures__ and paste these JSONs there.

You can extract hasCycles to a utils file.
There is a better hasCycles method in src/components/NetworkMSBN/helpers.js. You can use it but extract it first to a utils file and test it with a network.