Return Spectrums from MPS(::ITensor, sites) and replacesites!

It would be very helpful to return the Spectrums of the new bonds introduced in functions like MPS(::ITensor, sites) and replacesites!(::MPS, ::ITensor, range). This could be done with an API like:

s = siteinds("S=1/2", 3)
A = randomITensor(s...)
spectra = []
ψ = MPS(A, s; spectra! = spectra)

so that spectra would store the Spectrum objects for each bond of the MPS. Note the exclamation mark on the keyword argument to indicate that the keyword argument is being modified in-place (we could consider the same convention for DMRG observers).

This API style is discuessed in ITensor/ITensors.jl#500, and would be helpful for finishing up the n-site DMRG PR (#434) as well as for returning more information from the MPS gate evolution function apply(::Vector{ITensor}, ::MPS) (which probably needs a full-fledged observer).