Refactor to single edges between nodes.

[william-aaron-CFA]

In order to cut down on confusion for relationships between nodes, the Edge attributes and classification should be written such that only one edge between each node can encapsulate all relevant information using a single edge.

[william-aaron-CFA]

• relation: a strictly-defined classification string for how two nodes interact with each other. We intended to frequently query the database using these classes. Example, a relation would be 'writes' denoting how a CodeFile(File) node writes a DataFile(File) node.
• role: Secondary classification string with no input or format limitations, however we still intend to frequently search this string for secondary nuance. For example, a role such as 'updates' on the 'writes' relation will denote that the CodeFile(File) node also reads a given DataFile(File) node in order to write out the output. The purpose of this is to make general behavior and intention readily discoverable, while also allowing nuance to be recorded
• EdgeMetaData: Any edge-case crucial information which does not fit cleanly into this design can be recorded here.

Here, strictly-defined means that we have predetermined what each class means, and any value which does not fit one of these predetermined classes will throw a python error. Ensures database consistency.