[Suggestion] allocating thetas and labels to stratified parameters for calibration is not quite intuitive

[woldmuyn]

Description

First try at calibrating model with stratified parameters I used sublists for every parameter in side a list theta for my initial condition. After looking in the source code it was clear that the parameters had to be put in succession in one list.

Suggestion

Do a check for stratified parameters and do an automatic allocation and auto generate labels
Example:

stratification = ['x','y']
pars = ['par', 'strat_par']
theta = [theta1, [theta2, theta3]]
labels = ['$' + par + '$' for par in pars if par not in model.parameters_stratified_names] + ['$' + par + '_{' + strat + '}$' for par in pars for strat in stratification if par in model.parameters_stratified_names]

This is not necessity, but would be a nice feature, so that you only have to give the parameters to calibrate