Speed up apg

[marineLM]

• Create a C++ function grad_logistic.cpp for the logistic regression gradient
• Modifies gml.apg.R so that: 1) the gradient function taken is the C++ one and the R one, 2) the list opts taken by the C++ gradient grad_logistic.cpp has the design matrix as first element (This is required by grad_logistic.cpp)
• Changes the function used to compute isotonic regression in prox.boundednondecreasing.R (pava -> isoreg)

[marineLM]

With these changes, glm.apg.R calls a R function called grad_logistic.R. To make this function available to R, it need to be sourced with the following line: sourceCpp('path_to_cpp_function/grad_logistic.cpp'). So far this line does not appear anywhere so R won't know what grad_logistic.R is.