fix(lmweight):Fixed upperbound wrong bounds formu

Upper bound formula for TWO_STAGE and DIRICHLET should have doc_length_upper_bound
in numerator rather than doc_length_lower_bound. Since it sum was exceeding the bound.
Replace lowerbound in numerator with upper bound.
removed it from the xapian LMWeight class.

I am planning to keep it them as a local variable in init function ... one other option is directly call the function. but then the line will get clumsy.

I think better to have a local variable in init and then use them. Do you think otherwise ?