[alxbilger]

The objective is to provide a diffusion feature to be used for any type of element. It made sense to derive the class FEMForceField. This class is not tight to elasticity. However, it was developed with elasticity in mind and was not adapted to diffusion. For diffusion, we must make the difference between the field to integrate (temperature etc) and the spatial domain. For elasticity both are the same. It is already the case in MeshMatrixMass where there are two template parameters. So I used the same strategy: a new template parameter in FEMForceField class (breaking).

To use FEMForceField, I had to add the dependency to SolidMechanics.FEM.Elastic in Diffusion. In the future, we can think about moving the base class, in order to break this dependency.

The new diffusion force field gives the same result for the scene examples/Component/Diffusion/TetrahedronDiffusionFEMForceField.scn. The legacy force field, specialized for tetras is faster as an optimization specific to linear simplicials is used. To stay generic, the optimization cannot be applied in the generic class introduced in this PR.

Here are two videos of the introduced example. It shows similar results for both types of elements, hexas and tetras:

sofa_video_r60_0001.mp4

sofa_video_r60_0002.mp4

Questions:

• I added a lot of explicit template instantiations. It may increase the compilation time. Not sure all are necessary. To be discussed.

Note:
Although the PR is not based on any other PR, the example requires the following PRs:

• [Mass] Make MeshMatrixMass positive definite on hexahedra and quads #6410
• [Visual] Fix vertex value range in VisualMesh #6432

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