Add properties for neutron, hydrogen isotopes, helium isotopes

[RemiLehe]

This adds the values of the mass and charge for the following species:

• neutron
• deuterium
• tritium
• helium3
• helium4

The mass data is taken from:

• this NIST page for the neutron
• this NIST page for the isotopes of hydrogen
• this NIST page for the isotopes of helium

In the cases of the isotopes, I subtracted the mass of the electron, and converted to the units of proton mass, using:

M = (M_nist * m_u - m_e) / m_p

where m_u is the atomic mass unit, m_e is the electron mass, m_p is the proton mass.

[RemiLehe]

Note that this is not very consistent with the way in which we treat "helium".
For consistency, we should distinguish:

• hydrogen (includes the mass of the electron, averages over isotopes abundance)
• proton/hydrogen1 (does not include the mass of the electron, does not average over isotopes)
  However, in order to preserve the code behavior, I did not introduce this change.

[ax3l]

Agreed. Didn't we discuss this for #3090 that @lucafedeli88 has open? :)

[RemiLehe]

Ah, yes. Actually the present PR duplicates #3090. I will close it for now.