This class implements the BM weighting model, which generates the original
weight given by the BM25 formula, without frequency normalisation and query
Feel free to combine the BM model with any frequency normalisation method.
However, it is NOT recommended to use BM with the first normalisation for
after effect. For example, to use the BM model with the normalisation 2,
add the following line in file etc/trec.models:
DFRWeightingModel(BM, , 2)
Leave the space between the comas blank so that the first normalisation is