[Refactoring, DA] Better handling of analog blocks eigenvalues

[jpmoutinho]

Analog block eigenvalues are not defined because currently analog blocks only get converted into an actual operator when converted to HamEvo in the PyQ backend. GPSR requires these eigenvalues and currently there is a workaround where the pulser backend will call the PyQ backend analog block converter just to get the HamEvo and get the eigenvalues from there.