Display LF as tensors

[tmedani]

Hi @rcassani
In this discussion, I would like to highlight the idea of adding the leadfield display as ellipsoids, using the same function as view_fem_tensors .

For this purpose, in this branch , I created a more generic function "view_tensors" that can be called both from the FEM mesh to view tensord and also from the headmodel to view the LF [same as before] [I edited also the tree call back function].

For the fem mesh, this function displays the FEM tensors as ellipsoids or a main line at each FEM element centroid.
To adapt it to the head model, I convert the LF into the 3 eigenvalues and eigenvectors, so that at each source point we have a tensor [formatted similarly as the FEM tensors format, but now applications are at the source grid points].

Help wanted: the function works for the FEM mesh and not for the LF.

The final idea is to embed this visualization within the main function "view_leadfields_vector" with all the possible options, allowing changing the references, view channels, sources ..., ....

Thanks

[tmedani]

@rcassani, in the first place, if we can have the view_tensor work for the LF, it will already be a good achievement.
We can then just embed it within the view leadfield function.
My suspicion is that it is not working as of now because either the size of the ellipsoids or the coordinate systems.