[ENHANCEMENT] Add `eigendecomp`

[mtfishman]

Add eigendecomp as an abstraction on top of eigen, which would factorize a tensor into a product of the right eigenvectors, the diagonal matrix of eigenvalues, and the left eigenvectors (https://en.wikipedia.org/wiki/Eigendecomposition_of_a_matrix#Eigendecomposition_of_a_matrix). In general the left and right eigenbases are inverses of each other, but in the case where the map is hermitian they are conjugates of each other.