Add function ut_array_1d_int_mean.

The functions computes the mean of a 1D integer array.
Add function ut_color_palette.

The function fills out an array with a palette of colours.
The palette is made of 137 colours (comes from HTML I think).

The function takes two additional arguments: the minimum and maximum
average channel value that a colour must have to be recorded in the
output array.
Merge branch 'master' into doc

Conflicts:
	src/ut_array.h