feat(QuantumMechanics): Fourier conjugate of the momentum operator

[adambornemann-glitch]

Add §C to Operators/Momentum.lean: the Fourier transform conjugates the momentum operator 𝐩 i = -iℏ∂ᵢ into multiplication by the real symbol 2πℏ·ξᵢ (fourier_momentumCLM_eq), via 𝐩 i as -iℏ times the line derivative (momentumCLM_eq_smul_lineDerivOp). The symbol is packaged as momentumSymbol i with a @[fun_prop] temperate-growth lemma.

[github-actions]

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[gloges]

I've been mulling on this over the weekend and I'll be honest, I don't understand the ultimate purpose of this PR.
You've been building all of the machinery to get a great definition for any derivative operator,

def derivOperator (f : Space d → ℂ) : SpaceDHilbertSpace d →ₗ.[ℂ] SpaceDHilbertSpace d :=
  unitaryConj (fourierUnitary d) (ℳ volume f)

whose dense domain, adjoint, self-adjointness, etc. then follow from general properties of conjugation and mulOperator. Limiting ourselves to only momentum (linear f) in a funny way makes things both less generally applicable and harder to prove (unimportant details of f get in the way). Let's discuss and come up with a plan of attack on the Zulip.