qv_fitter start with offset circuits

[PetarJurcevic]

Currently, in order to add data and statevectors to the qv_fitter, the names need to start from 0: for example qv_depth_7_trial_0

However, for analysis and running a lot of circuits in batches, it would be nice to be able to the following:

job = backend.run(qobj)

qv_fitter=[]
qv_fitter = qv.QVFitter(qubit_lists=qubit_lists)
qv_fitter.add_statevectors(ideal_results[100:200])

qv_fitter.add_data(job.result())```


If I want to do this right now, I have to rename the circs, set the naming of circ[100].name = qv_depth_7_trial_0 and the same for ideal_result[100] in order to get the fitter to work.