Account for grain orientations in GBDiffusion material model

[strinner213]

• allow for arbitrary (anisotropic) bulk diffusivity tensor in input file
• read grain orientations from dataset in input file
• read GB normals from dataset in input file (not from json dict)
• add option to export grain orientations for visualization

Checklist:

• I made sure that the CI passed before I ask for a review.
• I added a summary of the changes (compared to the last release) in the CHANGELOG.md.
• If necessary, I made changes to the documentation and/or added new content.
• I will remember to squash-and-merge, providing a useful summary of the changes of this PR.