Diagonalization of the photo-absortpion tensor
It modify some output format for the case the sigma is diagonalized. It fix some problem in the diagonalization procedure that gives LAPACK errors. "* In deigensolve_nonh, LAPACK dgeev workspace query returned error message 32521"
It introduce a new variable PropagationSpectrumSymmetrizeSigma. Photo-absorption cross-section tensor should be symmetric. This variables ensure that removing numerical fluctuation.
- I have checked that my code follows the Octopus coding standards
-  I have added tests for all the new features added in this request.