Add Tensorboard logging for deep learning experiments

We add the tensorboard writer to log metrics that only concerns deep learning.

This MR introduces a split between two loggers:

• The melissa logger, which can be accessed through the logging library;
• The tensorboard.SummaryWriter, which is nested in the DeepMelissaServer.

The first one is used as a logger provided by logging library, while the second one is called with self.log.