Move-in C++ code from Bob; Add 3 more trainers

doc/conf.py

doc/index.rst

setup.py

xbob/learn/linear/cpp/lda.cpp

xbob/learn/linear/cpp/logreg.cpp

xbob/learn/linear/cpp/machine.cpp

xbob/learn/linear/cpp/pca.cpp

xbob/learn/linear/cpp/wccn.cpp

xbob/learn/linear/cpp/whitening.cpp

xbob/learn/linear/include/xbob.learn.linear/api.h

xbob/learn/linear/include/xbob.learn.linear/lda.h

xbob/learn/linear/include/xbob.learn.linear/logreg.h

xbob/learn/linear/include/xbob.learn.linear/machine.h

xbob/learn/linear/include/xbob.learn.linear/pca.h

xbob/learn/linear/include/xbob.learn.linear/wccn.h

xbob/learn/linear/include/xbob.learn.linear/whitening.h

xbob/learn/linear/lda.cpp

xbob/learn/linear/machine.cpp

xbob/learn/linear/main.cpp

xbob/learn/linear/pca.cpp