Reduce the computation time needed for bilateral upsampling 

I took a couple of exponential functions out from the innermost loops in order to reduce the computation time. The values for those exponential functions are now saved in a matrix (val_exp_depth_matrix) and a vector (val_exp_rgb_vector), and they are referenced inside the loops. The bilateral upsampling seems to work a lot faster now.

I'm new to github, so I apologize if there are some peculiarities in the commits. I read from the guidelines that there shouldn't be any merge commits, but I don't know how to get rid of them. Also tried to squash the commits into one, but again failed to do so.