PerturbativeTransmon eigenvalue decomposition should be independent from DuffingTransmon eigendecomposition when more than 3 energy levels are required.

[spherical-tensor]

This repo intends to use a perturbative expansion for computing eigenvalues as detailed in arXiv:1706.06566

However the computation is only implemented up to the second energy level ( via frequency and anharmonicity ). If a user requests higher energy levels then the code resorts to using the DuffingTransmon approximation. This is can lead to errors in higher level terms. A better strategy will be to fully implement the perturbative expansion for a PerturbativeTransmon as detailed in the reference and decouple hamiltonian dispatch for both these types.