Fork choice viz: finalized node shows 0 weight

[MegaRedHand]

As this image shows:

The visualization is showing the finalized block as having 0 weight. We should investigate why this happens.

[conache]

Hey @MegaRedHand, just dug into this and added more details + a proposed fix in PR #339.
It looks like the compute_block_weights() method returns 0 for the finalized block's weight. Since each node in the visualisation was filled proportional to the associated block's weight, that would mean that the finalized block would always show as empty. I fixed this in the PR.