Make Layer names refer to LayerKeys

[GoldenZephyr]

No description provided.

[nathanhhughes]

@yunzc This is the implementation of what we were chatting about earlier (where layer names now refer to LayerKey, not LayerId). The implication on the Hydra side is that you have to do something like:

const auto layer_id = graph.getLayerKey(DsgLayers::AGENTS)->layer;
const auto& layer = graph.getLayer(layer_id, 'a');

instead of

const auto& layer = graph.getLayer(DsgLayers::AGENTS, 'a');

to get the agents. Everything else you can just drop the extra partition ID if you're using a name. We'll probably merge this in soon rather than later, but lmk if you have thoughts

[yunzc]

Yeup, looks good to me!