Allow duplicate connections when initializing matrix wrappers

Created by: femtobit

The current matrix wrapper implementation assumes that each pair of indices (i,j) occurs only once when collecting the results of Hamiltonian::FindConn over all states using HilbertIndex.

This is not true in some cases, e.g., for a 2x2 cubic lattice with periodic boundary conditions, where FindConn currently returns two edges between each pair of adjacent nodes. With this commit, matrix elements for multiple edges are now added together. (We could alternatively disallow duplicate connections in the Hamiltonian as these might indicate an error in the model. Still, I think this case should either be prevented more thoroughly in the Hamiltonian implementation or the matrix wrappers should handle it.)

(This PR also includes a second commit, 19c668e2, which compresses the sparse matrix after setting up the coefficients. This should save memory.)