Refactor physical constants variables with dataclasses

[papantel]

_commons.py file contains several constants.

To manage this kind of constants in a clearer way, it is possible to define one dataclass by element (lead, bismuth, lbe), and one attribute by constant.

Example:

@dataclass(frozen=True)
class LeadConstants:
    melting_temperature: float = 600.6  # K
    ...

TODO : check if it’s as optimized as direct variables declarations.