Move away from Dataset <> SingleYearDataset conversions

[juaristi22]

Ideally, the calibration code would only use SingleYearDataset throughout. However, until -core is updated, Simulation objects are only compatible with the old Dataset class. Once -core is updated, I hope we can simplify _legacy functions and avoid class-type conversions.

[juaristi22]

Will be possible once issue #394 in -core is addressed.