Convenient constructors for tensor networks on grids

Currently to make a tensor network on a grid (a finite MPS/PEPS) the interface is:

using NamedGraphs
using ITensorNetworks
s = siteinds("S=1/2", named_grid((2, 2)))
ψ = ITensorNetwork(s; link_space=4)

or without site indices:

ψ = ITensorNetwork(named_grid((2, 2)); link_space=4)

It would be convenient to have some shorthands like:

s = siteinds("S=1/2", (2, 2))
# s = siteinds("S=1/2", 2, 2)
ψ = ITensorNetwork(s; link_space=4)

or:

ITensorNetwork((2, 2); link_space=4)
# ITensorNetwork(2, 2; link_space=4)

The notation siteinds("S=1/2", 5) for making a path graph of length 5 will conflict with ITensors.jl notation so for now that one may have to be siteinds("S=1/2", (5,)). Additionally, we should allow functions like ITensorNetwork accept Vector{<:Index} which gets output by siteinds("S=1/2", 5), and in general allow Vector{<:Index} act like an IndsNetwork with linear integer vertex names, like Vector{ITensor} should act like an ITensorNetwork with linear integer vertex names.

Start by inspecting the existing ITensorNetwork, siteinds, IndsNetwork, and Vector-related constructors described in the issue. Add the requested grid shorthand forms and compatible vector behavior, then verify that tensor networks and site indices can be created for the shown grid and path examples without conflicting with ITensors.jl notation.