Implement salinity and volumetric salinity

[acodcha]

Salinity can be measured as mass of salt per volume of water or in mass of salt per mass of water:

• PhQ::Salinity (dimensionless; mass of salt per mass of water)
• PhQ::Unit::VolumetricSalinity (mass of salt per volume of water)
• PhQ::VolumetricSalinity (dimensional; mass of salt per volume of water)