greater control over vertex size and placement in `network`

[Max-Bladen]

Is your feature request related to a problem? Please describe.

Users have had trouble with using the network() function as the vertices begin to overlap (see here). The vertices completely cover some edges, making interpretation impossible.

Describe the solution you'd like

Add simple controls to aid the user in influencing the network visualisation. This may include the size of the vertices and the distances between them.

Describe alternatives you've considered

Use of shape == "none" can rectify this issue in some cases. However, it doesn't seem to be sufficient in most instances.