Switch to logging for monitoring/printing training progress

[bartvm]

Using logging has some advantages, such as being able to redirect output to a file (in a more refined way than just dumping stdout), applying filters, silencing, etc. I came across this when I wanted to silence the output during tests.

[rizar]

While it is true that silenting logging is much easier, I am not sure that we should use it in 100% cases. The extension is called Printing, and the user expects the results in the output stream, not in the error stream. To achieve that when using the logging package additional actions are required.

Meanwhile I fully agree that everywhere outside this extension we should not use the print function.