feat(QuantumMechanics): basic properties/defs for HarmonicOscillator

[gloges]

The harmonic oscillator should probably serve as the model example of a well-developed, well-documented quantum mechanical system -- this PR lays the groundwork.

• Adds @[simp] tags to the hbar lemmas (requires a fix to the 1d QHO).
• Defines the HarmonicOscillator structure containing data for the mass and natural frequencies (all positive).
• Defines the characteristic lengths $$ξ_i = \sqrt{ℏ / m \omega_i}$$ and introduces some lemmas to take care of inverses and squares (to help with the sqrt).
• Defines a property IsIsotropic for when all natural frequencies are equal. When this property holds the potential is spherically symmetric: eigenfunctions can be written neatly in terms of spherical harmonics and the spectrum is highly degenerate.
• Defines kinetic, potential and Hamiltonian operators. Some of their important properties (e.g. (ess.) self-adjointness) depend on improving the definition of derivative operators (this is currently being developed).
• Creates HarmonicOscillator/LadderOperator.lean with a rough TODO list.

[github-actions]

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[jstoobysmith]

Looks good - approved.