Allow DataFit to take a list of simulations

[brosaplanella]

At the moment DataFit can fit a single Simulation to various datasets. We should extend the class to be able to take a list of Simulation as well, and in that case it would fit first simulation to first dataset, second to second and so on.

This would be helpful in cases where we want to fit the same parameter to data from various experiments.

[brosaplanella]

I think this applies to all optimisation problems. If we have a model (i.e. what we can't change) and data (i.e. what we can't change), then we can either have:

• Single model and single data: fit the model to data.
• List of models and list of datas: fit first model to first data, second to second, etc.
• Single model and list of datas: fit the model to all the datas.

There is a fourth option which is list of models and single datas. I am not sure if that would be a relevant case, but we could implement it if it is not a big deal.