simln-lib: Improve usage as a library

Using `simln-lib` as a library from another project, using this as a catch-all tracking issues for things that come up as I us it. If they're important enough, they can be spun out into their own issues.

- [ ] Implement `Default` for `SimulationCfg` (just nice to have)
- [ ] Consider publishing a `sim-cli` crate with parsing helpers that we've currently got in `main.rs` -> this would be very helpful for folks looking to get going quickly with their own custom simulations (I ended up just copying stuff)
- [ ] Library should probably allow user better control over logging