docs: improve split-operator documentation

[Lewiscowles1986]

I've added a high level description which I think better classifies and summarises this approach to solving a problem.

I've used the term meta algorithm, because this can be applied to a wide number of problems, with different, contextual and situational concrete algorithms.

[leios]

Wait, I might be confused here. So my understanding was that:

1. The split operator method almost always refers to what's presented in the text as is. That is, a specific method that splits up the operators in a hamiltonian to solve them in their specific spaces. The split-step Fourier method is the same algorithm under a different name
2. The split-step method (without the Fourier distinction) is as you say, a general class of algorithms that can be used in a variety of different situations.

[Lewiscowles1986]

Maybe I need to further improve this, and I've over-genericised the text.

If I'm honest, I was a little... What? while reading this, so I was trying to simplify from a process of collating a few sources.

• Accelerating the Fourier split operator method via graphics processing units
• OPERATOR SEPARATION OF VARIABLES FOR ADIABATIC PROBLEMS IN QUANTUM AND WAVE MECHANICS
• Noisy intermediate-scale quantum simulation of the one-dimensional wave equation
• Quantum Lattice Boltzmann is a quantum walk
• Hamiltonian formulation and symplectic split-operator schemes for time-dependent density-functional-theory equations of electron dynamics in molecules

I'd welcome feedback if I've made the text harder to understand, or misunderstood any of the materials, because I am not a quantumn theoretical computer scientist, or theoretical physicist