Support Weight Matrices

[PaulWAyers]

Currently, we allow weights for each dimension, and then use a (diagonal) weight matrix. In some applications in quantum mechanics, it would be useful to have a positive semidefinite weight matrix (which would be symmetric in all cases I can imagine). The idea would be to let the user pass a vector (then construct a diagonal weight matrix) or a matrix (then use the matrix).

In the cases I'm thinking about, the weight matrix would be the atomic-orbital overlap matrix or the one-electron reduced density matrix.