TriMesh enhancements

[kcpevey]

I have some suggested improvements/expansion of TriMesh that my work would greatly benefit from.

1. Ability to select an individual node and to see x,y,z information and any additional vdim data
2. Ability to select multiple vertices at once using a polygon selection (i.e. not just a box select).
3. Ability to select an element of the trimesh and see its material properties
4. Ability to color a trimesh based on the material property of the individual elements

We have discussed all of this in bits a pieces at various times, but I wanted to have all my thoughts and suggestions down in one place. Further discussion is anticipated.

[kcpevey]

@philippjfr This is a good start on a roadmap from a user perspective. Perhaps you can define a n equivalent roadmap from the implementation/developer side of things?