@window_filter(nim, coef:tab, pos:numeric)
build a new nim by processing each component independently.

Each component is the result of a smoothing process of the breakpoints. Ecah breakpoint of the new nim is computed by the dot product of coef with a sequence of y of the same length as coef where position pos corresponds to the current breakpoint.

For example,

       @window_filter(nim, [2], 0)
build a nim by scaling the image of nim by 2.
       @window_filter(nim, [0.1, 0.2, 0.4, 0.2, 0.1], 2)
is a moving weighted average with symmetric weights around y.

See also @filter_max_t, @filter_median_t and @filter_min_t.