Weighting Schemes -- Project Progress

Hello everyone,

I have been working on adding support for BM25+ weighting function from the
last couple of weeks. Initially, I considered modifying bm25weight.cc to
add support for BM25+ function without disturbing functionalities of BM25.
But that didn't work out very well. A day or two was spent trying to
refactor and debug the same code.

Later, I took another approach following the suggestions from James and
implemented a new sub class (BM25PlusWeight) for BM25+ weighting function.
All tests are passing. I've have pushed the changes to update the pull
request: https://github.com/xapian/xapian/pull/104

Next, I'll be refactoring the code, improving test coverage in
bm25plusweight.cc. and writing the documentation for BM25+.  By the mid-end
of the next week, I'll begin working on the implementation next weighting
function as per project timeline i.e. PL2+ weighting function.

Regards,
Vivek