Implement support and easy access to multi-step cycling experiments

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• Implement a "container" capable of handling multiple cell-cycling objects containing data points recorded at different current densities.
• Implement a loader/parser of the new battery-module format generated by Biologic instruments.
• Implement capacity retention across multiple experiments with fixed reference.
• Implement volume and electrode area to explicitly access volumetric and unit-surface data.
• Write tests and update the module documentation.

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Closed by #68