Warn color range clamping

[albertopasqualetto]

I encountered the problem described in #102 since I did not read the documentation carefully and I think that a warning should be shown when values are clamped, otherwise the results are not those expected (for surface meshes vertex coloring I get flat color faces instead of color gradients centered on the vertex).

I implemented an utility function to show a warning. I am a novice of this library, so I am not sure I did everything correctly. Also the code is not tested, but it is pretty easy.