Change/Improve values of vertex-type and edge-type constants

[clhunsen]

Description

The vertex and edge types as defined in util-networks.R are currently defined as magic numbers (historically coming from vector indices), but we should refactor them to be of type character. This way, we are able to actually "read" the vertex and edge attributes without any lookup.

https://github.com/se-passau/codeface-extraction-r/blob/34b9d35617e0d043d86a457b56d2ebc0a5ef02cd/util-networks.R#L31-L40

Proposed Solution

## vertex types
TYPE.AUTHOR = "Developer"
TYPE.ARTIFACT = "Artifact"

## edge types
TYPE.EDGES.INTRA = "Unipartite"
TYPE.EDGES.INTER = "Bipartite"

The solution should work out-of-the-box, so no changes to the test suite or any other parts of the code base are needed; that is why we use constants!

The solution is inspired by the plot module.

Possible problems

• The motif identification works on numerics for the vertex encoding (see here)
• The plotting module may need some treating, clean-up as well as fixes.

Versions

Any version as of now.

---

What is your opinion on the naming?

[clhunsen]

Fixed with the merge of #111.