Add configurable and generic Logging.

[chenkins]

Objectives
Flatland users should be able to add their own loggers,

• Add configurable logging (levels, modules) in RailEnv for domain-specific logging.
• Checkpointing as one implementation of Implement generic Callbacks. #76 ?
• Tensorboard logger as one implementation of Implement generic Callbacks. #76 ?

Todos
Needs to be refined/split.

Minimal testing requirements

• Show correctness of implementation

Additional context
Relates to #76