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Implement image.Image interface with FloatImg

This is done by having each FloatImg know two functions:
- ColorFunc
- ColorModelFunc

for converting the []float32 corresponding to a single pixel
to a color.Color and to determine the color.Model to use for the image
Adapt to new AtF
Convert to use the image.Image interface for FloatImg

also enables writing a color coded representation
image of the u,v vector field, currently the colors
are assigned by mapping u and v on different
values of the Cb and Cr channel of YCbCr image
and mapping the magnitude to nthe Y channel.