Double excited electronic states

[detrin]

Currently the system supports ground + singly excited electronic states (N+1 states for N molecules). Double excitations (two molecules excited simultaneously) are needed for nonlinear spectroscopy (2D spectra, pump-probe).

What needs to be implemented

1. Extended electronic state space — from molCount+1 to molCount+1 + molCount*(molCount-1)/2 states (ground + single + double excitations)
2. Double-excitation Hamiltonian blocks — couplings between single and double excitation manifolds
3. Extended aggregate construction — update AggregateCore, AggregateTools, AggregateOperators to handle the larger basis
4. Updated tracing — trace_bath and related functions must handle the new block structure

Impact

Architectural change affecting aggregate construction, Hamiltonians, and tracing. The electronicIndices function and indicesMap need extension. Consider a flag: Aggregate(...; max_excitations=1) defaulting to current behavior.

Proposed files

• Modifications to src/aggregateCore.jl, src/aggregateTools.jl, src/aggregateOperators.jl
• test/test_double_excitation.jl — tests