Output eigenvalue from ITensorTDVP.dmrg and dmrg_x

[mtfishman]

Output the eigenvalue along with the eigenvector from ITensorTDVP.dmrg and dmrg_x in a tuple.

The eigenvalue is returned as the first argument, following the convention of many eigensolvers in Julia, like LinearAlgebra.eigen, KrylovKit.eigsolve, Arpack.eigs, IterativeSolvers.lobpcg, and ITensors.ITensorMPS.dmrg.

User code like:

psi = dmrg_x(H, psi0; nsweeps=10, maxdim=100, cutoff=1e-6)
psi = ITensorTDVP.dmrg(H, psi0; nsweeps=10, maxdim=100, cutoff=1e-6)

will need to be changed to:

energy, psi = dmrg_x(H, psi0; nsweeps=10, maxdim=100, cutoff=1e-6)
energy, psi = ITensorTDVP.dmrg(H, psi0; nsweeps=10, maxdim=100, cutoff=1e-6)

@emstoudenmire @amilsted