Enhancement for Issue #233

[Max-Bladen]

This PR contains numerous changes to the network() function. Users noted that a lack of control over vertex size and distances made for difficulty in plotting. Vertices frequently overlapped, covering the edges completely - essentially nullifying the purpose of the figure. The following changes were made:

• Added graph.scale parameter: influences the scale of the entire network. Increasing this value essentially "zooms out" from the plot, decreasing vertex size and proximity.
  • Range: 0-1,
  • Default: 0.5 (network() functions the same as prior to PR)
• Added size.node parameter: influences the size of the vertices. Works independently from graph.scale.
  • Range: 0-1,
  • Default: 0.5 (network() functions the same as prior to PR)
• If user sets shape.node == "none", then the vertex labels will be coloured in the same fashion as the vertices would be.
• Cleaned up some of the checks for various parameter and ensured minmal chance of warnings/errors occuring when using network() on different objects
• Added tests to maintain coverage
• Updated documentation and SemVer