[ENHANCEMENT] Expose `truncerr` in calls to `truncate!`

Hello,

I'd be interested on thoughts here. I think it would be useful to be able to get the truncation error corresponding to a call to truncate!.

Currently truncate! as defined in abstractmps.jl only returns the MPS, but each truncation that is performed is a call to svd which returns the TruncSVD type which contains the truncation error.

Maybe one solution then would be to allocate a small vector with as many elements as there are bonds of the MPS. Then in the right to left sweep just collect the truncation error into each element and have that be optionally returned (could make it a kwarg).

If this functionality is already available then please just disregard and close this feature request.

Here is some psuedocode for what I had in mind (I've not actually tested this to see if it works, just what I was thinking). If this is something that sounds reasonable, I can prepare the pull request. I can make the corresponding updates to truncate as well.

function truncate!(
  ::Algorithm"frobenius", M::AbstractMPS; site_range=1:length(M), return_truncation_error=false, kwargs...
)
  N = length(M)
  truncerr = zeros(N-1)

  # Left-orthogonalize all tensors to make
  # truncations controlled
  orthogonalize!(M, last(site_range))

  # Perform truncations in a right-to-left sweep
  for (idx, j) in enumerate(reverse((first(site_range) + 1):last(site_range)))
    rinds = uniqueinds(M[j], M[j - 1])
    ltags = tags(commonind(M[j], M[j - 1]))
    U, S, V, spec = svd(M[j], rinds; lefttags=ltags, kwargs...)
    truncerr[idx] = spec.truncerr
    M[j] = U
    M[j - 1] *= (S * V)
    setrightlim!(M, j)
  end
  if return_truncation_error
  return M, truncerr 
  else
   return M
   end
end

Start in abstractmps.jl with truncate! and trace how each svd call produces a TruncSVD value containing truncerr. Compare the related truncate implementation and determine how an optional return value should preserve the current default behavior. Done means both truncate! and truncate can expose per-bond truncation errors without affecting existing callers.