[ITensors] [ENHANCEMENT] Easy padding of an MPS to a desired bond dimension.

Is your feature request related to a problem? Please describe.

For e.g. 1-site DMRG (or DMRG-X), starting from an initial product state is viable if the MPS is first padded to the desired final bond dimension. This can be more efficient than 2-site DMRG(X), e.g. if the site dimensions are larger.

Describe the solution you'd like

Something like pad(mps, maxdim) that defaults to padding the tensors with zeros. I realize this might be trickier with nontrivial quantum numbers, but I would be happy with just the dense case for now.

Describe alternatives you've considered

Adding the original MPS and a zeroed MPS for padding only works in some cases. For example, this does not work if I want to pad some bonds but not others, as adding an MPS to another with some bond dimensions = 0 currently fails, and there is currently no easy way to initialize an empty MPS with nonuniform bond dimension (In MPS(sinds; linkdims), linkdims can only be an integer right now). This also seems a rather roundabout way to achieve the padding (e.g. I have to compute the difference of each bond dimension to the target dimension).

Start by examining MPS(sinds; linkdims), especially its current integer-only linkdims behavior, along with the MPS construction and addition paths described in the issue. Done means a pad(mps, maxdim) entry point can zero-pad dense MPS tensors to the requested bond dimensions, including nonuniform bonds where applicable.