Mimic the Counter interface

[piskvorky]

Wrap the low-level implementations (min count sketch, hash tables for key iteration...) in a Counter interface. The goal is to make Counter <-> Bounter switch trivial for users.

Methods that cannot be implemented due to algorithmic constraints (key iteration with min count sketch etc) should raise a NotImplementedError with a clear error message.

[piskvorky]

@isamaru what's the status here? Are we raising NotImplementedError consistently?

If not, can you list what needs to be done, so others can take this over?

[souravsingh]

I would be interested to pick this up. We just need to refactor the API to make it similar to Counter, right?

[menshikh-iv]

@souravsingh yes, but please finish your current sLDA project first.