feat(QuantumMechanics): Begin spectral theory of symmetric operators

[gloges]

Begins the spectral theory for symmetric operators with some results on the regularity domain and numerical range.

• Adds some general results for behavior under negation / scalar multiplication.
• Defines realNumericalRange which is the numerical range of a symmetric operator as a subset of ℝ rather than of ℂ, allowing for the notion of upper/lower bounds.
• Proves that the regularity domain contains all complex numbers with nonzero imaginary part as well as (-∞,m) or (m,∞) when m is a lower/upper bound on the numerical range.
• Adds a characterization of when the regularity domain is connected.

[gloges]

t-quantum-mechanics

[github-actions]

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[jstoobysmith]

Looks good to me. Approved - will merge shortly.