WIP: FieldMatrix eigenvalue improvements
This MR is intended to add some performance improvements by adding specializations for eigen vector calculations of 1x1, 2x2 and 3x3 matrices.
As a start, we had to
- improve the tests
- fix a bug in the 3x3 ev calculation
We further
- added tests fr eigenvalue/eigenvector pairs
- added specializations for the eigenvector calculation for 2x2 and 3x3 matrices
Edited by Christian Engwer