Logger eval mode

The logger should have an option to either override the filename (for performance logging at least) or haven an explicit eval mode to separate performance data from training and eval

No file, test, or entry point is named in the issue. Start by locating the logger and its handling of performance, training, and evaluation data; done requires an agreed mechanism for overriding the filename or explicitly separating evaluation data, with the behavior covered by appropriate tests.

Written by the indexing model from the issue text.