Log print statements

[elbeejay]

We print to stdout with reckless abandon throughout the codebase. I think we should create functionality to optionally suppress these print statements (maybe a "quiet" flag or a "verbosity" argument), and/or we should also support printing any messages out to a .log file instead. This way they won't disappear with the terminal, and can be preserved with the particle walk data for review during post-processing or when the results of the simulation are later analyzed.