Resolve ownership of edge weight information between edge and connections classes

[stiber]

Right now, this is tied too deeply into the ConnGrowth class, due to historical reasons and the fact that ConnGrowth needs to update the edge weights. But it perhaps makes sense for the edges to own their weights. Moreover, for other connections classes, such as ConnStatic and STDP simulations, the edges update their own weights. Also, I think that the different apparent weight matrix data actually holds different values: one may be just area of overlap during a growth simulation, while the other has signed and scaled down actual weight values.

There is of course an intersection with the GPU implementation, and the GPU implementation needs to mirror the CPU implementation (including any modifications). This should also take into account the ability to copy such information back and forth between the host and device.