Update state config to be nested

[nreinicke]

Right now the state config requires us to put the state key at the top of the toml file since if it's not then it will get nested inside another section. For example if we have the config:

[graph]
edge_list_input_file = "../data/tomtom_denver/edges-compass.csv.gz"
vertex_list_input_file = "../data/tomtom_denver/vertices-compass.csv.gz"
verbose = true

state = [
    { distance_unit = "kilometers", initial = 0.0 },
    { time_unit = "minutes", initial = 0.0 },
]

state would get nested under the graph key.

We could update this to have a nested entry so it could be placed anywhere in the toml file, something like:

[state_model]
state = [
    { distance_unit = "kilometers", initial = 0.0 },
    { time_unit = "minutes", initial = 0.0 },
]

[robfitzgerald]

We no longer rely on injecting state features at config time