Convert experience ID to string

I suggest to propose converting experience IDs to strings. Advantages of string identifiers:
- we can have different names for different scenarios
- class-incremental "E0", "E1", ...
- task-incremental "T0", "T1", ...
- OCL "T0E0", "T0E1", where T indicates the "big" experience and E is the position in the stream

This change would be helpful for metric/logging because now we can use more relevant names. It also makes it more obvious that experience IDs should not be used as "task labels" during training or evaluation (they are only for logging).

Let me know what you think.

In practice, the change should be quite easy to make, it shouldn't break anything.