Update axis buffer and allow different curvature scalars for each edge

[JackDevine]

When you create a custom graph layout, you sometimes want the nodes to fall all have the same x/y coordinate. The old code for the axis buffer didn't behave nicely in this case. Additionally, the old code forced you to have the same axis buffer for each dimension. This is fine for most graph layouts that we generate, but it can be problematic with custom layouts (e.g. flowcharts).

I also added the possibility to have different curvature scalars for each edge, which I think is really useful for flowcharts, but also seems handy in general.

Example

using Plots, GraphRecipes
using LightGraphs

default(size=(400,400))
g = DiGraph(5)
add_edge!(g, 1, 2)
add_edge!(g, 2, 3)
add_edge!(g, 3, 4)
add_edge!(g, 4, 5)
add_edge!(g, 5, 3)
names = [:Thing, :Thingy, :Thinger, :Things, :Stuff]
x = [0,0,0,0,0]
y = [5,4,3,2,1]
curvature_scalar = zeros(length(names),length(names))
curvature_scalar[5,3] = 1.0
graphplot(g, x=x, y=y, names=names, curvature_scalar=curvature_scalar,
          axis_buffer=(1.1,0.2), w=2, color=:black, nodesize=0.3, nodeshape=:circle)

cc: @caseykneale

[caseykneale]

Awesome! Thanks for working on this Jack! I'm a little new to this collaborative stuff using Github, but let me take a crack at using PR. This will probably help me a lot with https://github.com/caseykneale/Sherlock.jl