Relax method for sigma from Real to Number and order eigenvalues at the end of the computation

[albertomercurio]

As title

[Copilot]

Pull Request Overview

This PR introduces two key improvements to the eigenvalue solver:

• Adds explicit sorting of eigenvalues and eigenvectors in the _eigsolve function to ensure results are ordered according to the sortby and rev parameters
• Broadens the type constraint for the sigma parameter from Real to Number to support complex shift values in eigenvalue problems

Comments suppressed due to low confidence (2)

src/qobj/eigsolve.jl:283

• The documentation string still shows sigma as Union{Nothing, Real}, but the actual function signature now accepts Union{Nothing, Number}. Update the documentation to reflect that sigma can accept complex numbers.

        sigma::Union{Nothing, Real}=nothing,

src/qobj/eigsolve.jl:298

• The documentation parameter description shows Union{Nothing, Real}, but should be updated to Union{Nothing, Number} to match the function signature and indicate support for complex shift values.

- `sigma::Union{Nothing, Real}`: the shift for the eigenvalue problem. Default is `nothing`.

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