Make loggers accessible from a create GridWorld

[jwaa]

Is your feature request related to a problem? Please describe.
When you created and ran a world with loggers, you often want to do some post-processing. For this it is handy that you can access the loggers that world used. However, this is not possible. Because 1) you pass a class of a logger to the grid world which instantiates a new one for every created world, and 2) because GridWorld does contain the logger instances but as a private object requiring the ugly work around of doing GridWorld._GridWorld__loggers

Describe the solution you would like
I want GridWorld.__loggers to be a Python property that makes it accessible to the public but without a setter so they cannot be altered.

Describe alternatives you have considered
The ugly workaround of acessing a private attribute using GridWorld._GridWorld__loggers

Additional context
Originally identified by @Wouter1 while working on the TuD version of BW4T (see question #267)

[Wouter1]

@jwaa Thanks for picking this up!
I'd like to add a few things

1. I in particular need the log filename that was created, so that I can read the log file contents. Even if I have the logger (I can get it with gridworld.loggers[0]) I still can not get to the filename
2. You can not access private instance variables. Yes internally they seem to get the name _GridWorld__file_name etc, but python does not allow accessing them from outside. Or maybe I need another trick for this to get around this

[Wouter1]

And one more thing, maybe relevant. I'm processing after every builder.run so it's not a real problem that the builder creates the loggers. As long as I can get the log file(s) that are made I think I can manage.

[jwaa]

The builder creates the loggers yes, but does not store them. This is done by the GridWorld in GridWorld.__loggers. Hence the request to make them accessible as GridWorld.loggers so you don't have the hassle of the double underscore.

Regarding your two bullets; see my suggestion in #267 to make the file name a publicly acessible property. If you are opting to do that by default; good idea that would be a different feature request :)