The DFR  PL2 Weighting Scheme 

I have added the collection frequency statistic to the Xapian::Weight class so that it can be accessed by the subclasses.I created a new map for this statistic.I've also tried my best to make the documentation neat as I've learnt from the previous feedback's Ive got.

Have added simple tests for the scheme.Will add feature tests after working on the feedback.

The base of the log is 2 everywhere as I've decided to follow the paper on DFR schemes by Amati rather than his thesis because a base of log 2 makes more sense as the normalized wdf will be wdf for a document of average length if we use a base of 2.  

Future DFR schemes will be easier to implement after I learn from the feedback of this scheme.