documentVector: Store tf instead of tf.idf

We need the tf for the LM.
inverter: Don't compute tf.idf

This is no longer required for the LM and thus unnecessary
indexer: Compute collection frequency

This is needed for the language model.
indexer: Count number of tokens
indexer: Initial LM based search
indexer: Implement LM with fixed lambda
indexer: Move lambda to constant and add a comment explaining the
choice.