quantarhei interoperability

[detrin]

Enable data exchange with quantarhei, the Python package that inspired OpenQuantumSystems.jl. This allows users to leverage both ecosystems.

What needs to be implemented

1. File-based interop — define a common JSON/HDF5 format for aggregates, Hamiltonians, and spectral densities that both packages can read/write
2. Import quantarhei Aggregate — read quantarhei's serialized aggregate data and construct an OpenQuantumSystems Aggregate
3. Export to quantarhei format — write OpenQuantumSystems data in quantarhei-compatible format

Alternative approach

Use PyCall.jl for direct Python interop, but file-based exchange is simpler and more portable.

Proposed files

• src/quantarhei_io.jl or ext/QuantarheiExt.jl — import/export functions
• test/test_quantarhei_io.jl — round-trip tests with sample data