replace discriminator::sum_outputs by sum_pos and sum_neg

discriminator::sum_outputs is returning the sum of positive outputs by
row. Likewise I noticed I needed sum_neg for
implementing (tp-fp)/positive (which is better to represent in bscore,
something I'm working on). It also simplify codes in discriminator and
inherited classes at a liekly no cost (MOSESUTest isn't slower andit
makes sense it should have negligible cost).
Fix CMake files for install
Add discriminating_bscore::full_bscore

That flag is for computing the bscores inheriting
discriminating_bscore at a finer grain.

Specifically, if enabled, the variable componant of the
discriminating_bscore is divided across all rows of the ctable. Each
one accounting for its contribution to the overall score.

This has been added to handle diversity pressure with
discriminating_bscore. By default it is disabled but once it has been
measured as yielding no overhead (which I believe) it will be enabled.

Also, only prerec_bscore currently takes advantage of it.
Forgot opencog/learning/moses/scoring/CMakeLists.txt
Merge pull request #125 from ngeiswei/bscore_prerec

Bscore prerec