Implement eigenvalue and eigenvector solver

[acodcha]

Implement a simple solver for eigenvalues and eigenvectors of a 3x3 matrix for the PhQ::Dyad and PhQ::SymmetricDyad classes.

References:

• https://en.wikipedia.org/wiki/Cubic_equation, in particular: https://en.wikipedia.org/wiki/Cubic_equation#General_cubic_formula
• https://stackoverflow.com/a/13328773
• https://www.calculatorsoup.com/calculators/algebra/cubicequation.php