[CollisionModel] New: added collision layer property

[polyakovkrylo]

This PR introduces collision layers, a collision filtering algorithm similar to the one used in Unity's collsion pipeline. The overall idea is to omit collision detection for models that are in different layers. This way, each layer can be used for different functionality, e.g. cutting simulation.

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[alxbilger]

Hey! Can you explain how it is different from the group property? Thanks

[polyakovkrylo]

Hi, @alxbilger! It works the opposite way the collision groups work. With collision groups, collision can only occur between the models' group sets do not overlap (no common groups). With the collision layers it' s the opposite -- collision can only occur if the models are in the same layer.

[alxbilger]

Suggested change

	Data<int> layer;
	Data<int> d_layer;

[alxbilger]

It also needs to be initialized in the constructor

[alxbilger]

Does it make sense to define more than one layer for a collision model?

[bakpaul]

Hi vlad, this PR has also been discussed during the dev meeting.
The conclusions are the same as for the #4226 : we need more context/example of use (for instance to see how you use it in your scenes or plugin).

We are also curious to know how you handle differences between this and the group mechanism : what happens if you are in the same time in the same group and in the same layer ? Are the two mechanism really necessary ?
It could be also possible to remplace the group mechanism by yours to have a more positive mechanism (same layer = collision instead of diffrent group = collision). But for this it would need to handle multiple layer by object.

And finally, again, some unit testing would be great if we manage to find a mechanism that suites every one.