Question regarding resistive distance calculation

[vboussange]

Hey there, I really appreciate all the work and tricks put into this nice package! :)

I have two questions regarding the implementation of the resistive distance:

1. The Laplacian matrix of the graph associated with the electrical circuit is singular; this is why the resistive distance is usually defined with the Moore-Penrose pseudo inverse of the Laplacian. But you use another trick to be able to calculate the voltages when injecting one amp into the system, possibly related to

   Circuitscape.jl/src/core.jl

   Line 152 in 4b6dbee

   matrix.nzval .+= eps(eltype(matrix)) * norm(matrix.nzval)

   Can you justify what is this line doing?
2. I am also confused with these two lines:

   Circuitscape.jl/src/core.jl

   Lines 218 to 221 in 4b6dbee

   	v .= v .- v[comp_i]
   	# Calculate resistance
   	r = v[comp_j] - v[comp_i]

   Why would r = v[comp_j] - 2 * v[comp_i] instead of r = v[comp_j] - v[comp_i]?

Thanks a lot in advance for your answers!