STYLE: use constexpr for static const char strings

src/OpenFOAM/global/constants/atomic/atomicConstants.C

src/OpenFOAM/global/constants/atomic/atomicConstants.H

...obal/constants/electromagnetic/electromagneticConstants.C

...obal/constants/electromagnetic/electromagneticConstants.H

...OAM/global/constants/mathematical/mathematicalConstants.H

...obal/constants/physicoChemical/physicoChemicalConstants.C

...obal/constants/physicoChemical/physicoChemicalConstants.H

src/OpenFOAM/global/constants/universal/universalConstants.C

src/OpenFOAM/global/constants/universal/universalConstants.H