Histogram computation uses reduce pattern

In previous versions, histogram computation was done using atomicAdd.
atomicAdd is not the best thing to use in order to speed-up a kernel.

The original kernel 'kernel_image_DalalTriggsHOGdescriptor' has been splitted into two smaller ones
in order to apply a reduce-like pattern.

The first kernel 'DalalTriggsHOGdescriptor_precompute_histograms' is responsible to compute the values
of d_h[y][x][bin][unique_id]. Each element of d_h can only be reached by one thread.

The second kernel 'DalalTriggsHOGdescriptor_reduce_histograms' sums up on unique_id.
The result is stored into: d_h[y][x][bin][0].
d_h[y][x][bin][0] = sum over unique_id of d_h[y][x][bin][unique_id]