Refactor MPSTensor construction to fix type piracy

[VinceNeede]

Closes #471.

Not really my place to be doing bigger refactors yet (this is only my second PR, after my
DMRG3S implementation), but the discussion in #471 pointed pretty directly at a concrete fix,
so I wanted to give it a try rather than just patching zeros! → zerovector! and leaving
the type piracy in place.

What this does

• Removes the old MPSTensor(f, T, P, Vₗ, Vᵣ) / MPSTensor(::UndefInitializer, ...) /
  MPSTensor(d, Dₗ, Dᵣ) constructors, rather than just fixing the zeros! call inside them.
  As @lkdvos noted, these were type piracy (MPSTensor is a const alias for
  AbstractTensorMap, not a type MPSKit owns), and the original issue was really just a
  symptom of that.

• Introduces a small MPSMapSpace{S, Sₚ} struct that holds (Vₗ, P, Vᵣ), so the
  Vₗ ⊗ P ← Vᵣ leg convention lives in exactly one place instead of being repeated in every
  constructor/accessor.

• Extends Base.rand/Base.randn/Base.zeros for MPSMapSpace, giving essentially the
  zeros(T, Vl * P, Vr)-style interface you mentioned as a preferred option, e.g.:

  julia> zeros(Float64, MPSMapSpace(ℂ^10, ℂ^2, ℂ^10))

  These are added as genuine Base.rand/Base.randn/Base.zeros methods (not a new type
  alias constructor), so there's no piracy here — MPSMapSpace is a type this package owns.

What this does not do

I left out a dedicated mpstensor(...) convenience function for now. MPSTensor was never
exported, and grepping both MPSKit (src, test, docs, examples) and PEPSKit.jl for
MPSTensor(/MPSTensor.( turns up zero call sites outside the old constructor definitions
themselves, so I don't think there's a hidden dependency on the old constructor shapes. Given
that, it felt safer to start minimal and let you decide whether a convenience wrapper on top
of MPSMapSpace is actually wanted, rather than guessing at a signature nobody's using yet.

Happy to add mpstensor (or rename MPSMapSpace to something else, if Vl * P composite
notation is what you'd rather standardize on) if that's preferred — this is very much open
to being redirected.

[codecov]

Codecov Report

❌ Patch coverage is 60.00000% with 4 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/states/abstractmps.jl	50.00%	3 Missing ⚠️
src/states/infinitemps.jl	66.66%	1 Missing ⚠️

Files with missing lines	Coverage Δ
src/states/finitemps.jl	93.72% <100.00%> (+8.11%)	⬆️
src/states/infinitemps.jl	80.14% <66.66%> (+5.14%)	⬆️
src/states/abstractmps.jl	54.79% <50.00%> (+4.79%)	⬆️

... and 65 files with indirect coverage changes

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