Make a loggers filename publicly accessible

[jwaa]

Is your feature request related to a problem? Please describe.
For post-processing of the data a logger recorded, it helps to know where the data was stored. This is now hidden in GridWorldLogger.__file_name. It would be nice to have this as a publicly available attribute.

Describe the solution you would like

class GridWorldLogger:
    ...
    @property
    def file_name():
        return self.__file_name

Describe alternatives you have considered
None really...

Additional context
Originating from #267 and #268