Calculates the eigenvalues of the Adjancency Matrix, where the Adjacency matrix is a square matrix indicate whether pairs of vertices are adjacent or not on a graph - here, entries in the matrix indicate connections between nodes (and betweens nodes and tips). Entries in the adjacency matrix are weighted by branch length. Then, using the adjacency matrix, we calculate the spectral properties of the matrix, e.g. the minimum and maximum eigenvalues of the matrix. When the R package RSpectra is available, a faster calculation can be used, which does not calculate all eigenvalues, but only the maximum and minimum. As such, when using this option, the vector of all eigenvalues is not returned