[3dprint] filament gradients

I have been working on a way to blend two filaments together to achieve smooth gradients along a print surface. It is not perfect yet, but here is a brief description of this idea.

When working on our 'clean colors' technique, we made the observation that alternating layers of two filaments would produce different gradients of colors along the sides of the object. What happens is that the filament closer to the observer has more impact visually, than the one slightly behind. Interestingly, this paper made the exact same observation and came up with a very interesting way of depositing filaments to mix colors.

Starting from the same observation I came up with a different idea. Instead of changing the geometry I play with the flow of plastic. I do not alternate filament each layer but instead, at each layer, I deposit plastic twice (once for each filament) with a different flow of plastic. The two flows add up to 100% and by changing the percentage of each deposition you get different blends of colors. It is important though to always start by the filament having the highest percentage. What happens is that you get 'big' and 'small' cross sections of filament. And of course, the big one becomes more visible.

This works quite well and has interesting properties:
- the layer height is unchanged
- there is no change to the geometry of the print
The main drawback is that this requires carefull calibration to align the heads. This can be done in software by printing calibration tests. I have done initial experiments with spatial variations, but I yet have to explore this further. I am also curious to see what happens when doing this with soft/hard filaments and filaments of different properties.

If you experiment with this please let me know! If that works well enough we will include this feature in IceSL.