Add query strategy: QueryByCommittee

[chschroeder]

Feature description

The "query by committee" strategy should be available, and should be configurable to work either with vote entropy or KL divergence as selection criterion.

Pointers:

• Paper reference: A. McCallum and K. Nigam. “Employing EM and Pool-Based Active Learning
  for Text Classification”. ICML 98.
• Start in a new file, e.g. small_text/query_strategies/commitees.py

Expectations:

• The strategy should

Motivation

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Additional comments

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