Add multi-dataset optimisation routines

[BradyPlanden]

Feature description

Refactor the dataset class to support multiple datasets with the optimisation workflows able to support either serialised or parallel fitting / design optimisation for the combined datasets. Additional improvements would include a weighting function for the datasets.

Motivation

This functionality would enable design optimisation for varying operating conditions (fast-charge, CC-CV, GITT, etc.) with different initial conditions.

Possible implementation

• Refactor dataset class for multi-instance
• Update optimisation class with loop or vectorisation for instances in dataset
• Sum cost across datasets with applied weighting function
• Proceed with current optimisation loop

Additional context

No response

[BradyPlanden]

See #238