Should density items have the lower enumeration range of 0.0 instead of 1.0?

[vaitkus]

In issue #495 @nautolycus suggested that the enumeration values in dictionaries should preferably "<...> be confined to values that are physically possible rather than what might be thought "reasonable" (leaving the latter to validation applications or overlays) <...>". Based on this comment, the lower enumeration range of cell length items was changed back from 1.0: to 0.0:.

Maybe it would then also make sense to change the enumeration ranges of the _chemical_formula.weight_meas and _chemical_formula.weight from 1.0: to 0.0:, especially since the closely related data items _cell.atomic_mass, _exptl_crystal.density_diffrn and _exptl_crystal.density_meas already have the enumeration range of 0.0:.

Note, however, that the DDL1 dictionary also had the lower range of _chemical_formula.weight_meas and _chemical_formula.weight at 1.0:.