Top-level SimState

[janosh]

add from torch_sim.state import SimState in torch_sim/__init__.py. makes SimState accessible as

import torch_sim as ts

ts.SimState

for better ergonomics. related PR (as in should maybe have been part of) #52

[CompRhys]

I don't really have a strong opinion here and this isn't a breaking change as far as I understand as it adds a convenience import so LGTM.