Lbh15

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- fixed known bugs

- peer-reviewed documentation

- refactored property classes

- improved cross-platform path handling for user-defined properties

- inserted contributing guidelines

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- implemented thermo-chemical data and properties (solubility and diffusivity of impurities, Oxygen concentration range where protective oxide layer formation is assured,
entropy, specific enthalpy, Gibbs free energy, Oxygen partial pressure)

- improved performance by storing properties within dedicated attributes of liquid metal classes

- fixed known bugs

- implemented a tutorial where thermo-physical and thermo-chemical properties of liquid lead are needed for controlling the Oxygen concentration

- updated documentation to include thermo-chemical properties description

- included *Learn More* section into the documentation: it treats the properties related to the Oxygen concentration by the theoretical point of view and by providing
the description of the above-mentioned tutorial

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- constant variables substituted with the ones from :code:`scipy.constants`
- inserted :code:`pyproject.toml`

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- refactored the collocation of some already implemented methods

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- properties made dependent on the complete thermo-dynamic state (added pressure dependency)

- inserted functionality for computing upper and lower bounds of physical correlations

- implemented :code:`range_warning` decorator for verifying whether the value of the property is required within its temperature validity range

- properties handling moved from class level to instance level

- constant variables moved into a dedicated module