Implement logging

[jimbojw]

The inner workings of Megaplot are complex. Operations are batched and pipelined. Long running tasks are suspended and resumed.

Currently, there is no internal logging to signal when and how these operations progress. For the API user, this should make no difference, but for debugging during development it means that often temporary console.log statements are needed to elucidate what's going on.

Add logging to Megaplot in such a way that key operations can be logged to the console, but conditioned on some kind of log-level flag.