Migration to pathlib and cache groups for better end-user experience

[fedebenelli]

Main modifications:

• Now the paths for to_clapeyron work using the logic of Python's pathlib, which is a more portable way of defining paths to directories and files.
• The instantiate_mol_object can now be cached to have a better experience working with notebooks

[SalvadorBrandolin]

Excelent, thank you. The pathlib use must be applied also in the module "models" (by me, just a reminder)