Allow suppression of output to stdout when filtering

[matt-graham]

Also, can the new tests be made silent, to avoid all the

Writing output at timestep = N

messages?

Agree that these are annoying - I would personally prefer to remove all the println statements altogether or at least make them optional with a quiet flag or similar as there isn't any easy way to selectively disable at the moment that I can see. Or alternatively it looks like using macros from the Logging module would be better as then we can selectively disable output for different levels. But this seems a change that would probably be better in a separate PR.

Originally posted by @matt-graham in #244 (comment)