Error in SequentialGP::gradientEvidenceUpperBound (wrong length scales gradient) about gptk HOT 1 CLOSED

==Fix==
After writing down the gradient, we should have:

 W = (W-(W + (KB * (C + outer_product(Alpha, Alpha)))) * backslash(KB_new, KB)) *
backslash(KB_new,W);

instead of:

 W = backslash(KB_new, W - (W + (KB * (C + outer_product(Alpha, Alpha)))) * 
backslash(KB_new, KB));

This gives the correct gradient for the length scale. I'm not sure why. What we
compute in the end is the trace of the expressions A*inv(B)*C (works) and 
inv(B)*A*C
(doesn't work). I think both traces should be identical? Maybe it's some 
numerical
issue which depends on the order in which we multiply the matrices?