simln-lib: Improve usage as a library

[carlaKC]

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