gyromagnetic_anomaly

[DavidSagan]

Suggested names:

gyromagnetic_anomaly - For (g_spin-2)/2
g_spin - For 2mmu / (q * S) where m is the particle mass
g_nuclear - For 2* m_p / (e * S) where m_p is the proton mass.

[rot4te]

the magnetic moment \mu here is not already defined for arbitrary particles in the package; should it be?
I'm currently defining g_spin in terms of gyromagnetic_anomaly, which seems a little circular.