Rethinking default behavior for number of iterations

[MilesCranmer]

Something I have noticed is that many users expect the result of .fit(X, y) to be converged for the default parameters, and are surprised that running it a second time results in a different equation. This is a reasonable assumption given that converged solutions are the typical behavior for other ML models or optimization routines!

So, I think the default behavior for PySR should be a bit more clear that "convergence" is not really well-defined for symbolic regression. Perhaps we could leave the same behavior, but basically show a warning to the user if niterations is not set explicitly, such as:

No stopping criteria given, so will exiting after 40 iterations. Increase `niterations` to search for longer.

This seems like a simple solution which would make it clear to users that they shouldn't expect the equations to be converged.

We might also think about renaming niterations to max_iterations to be more clear about this. We should make this in line with the other stopping criteria: https://astroautomata.com/PySR/api/#stopping-criteria