Annotating true negative (TN) nodes in napari

[KristinaUlicna]

When annotating the image in napari, the annotation tool allows you to do the following:

• true positive (TP) edges (green) between nodes that belong to the same object
• true negative (TN) edges (magenta) between a node which belongs to an object and a random node (noise)
• true positive (TP) nodes (un-coloured, white) which are parts of objects
• true negative (TP) nodes are not accounted for

There is currently no way to explicitly annotate true negative nodes, as one can only draw over real objects. All unannotated nodes are currently left alone with a label 'Unknown', resulting in no information about the TN nodes at GCN train time, which leads to overfitting towards the TP class.

As a design choice, we decided to implicitly label all protruding nodes which form a TN (magenta) edge as TN nodes. The rationale is that the nodes which are a part of an object (even a different one) would themselves be labelled as TP within their respective object.

The implementation in code is as follows:
ADD PERMALINK

[KristinaUlicna]

Closes #117