Introduce mass

[cortner]

I think the default mass of a particle should be mass and not atomic_mass.

First, this does not create any ambiguities: The prefix atomic_ is to emphasize it is the mass of an atom. If the particle of which we are getting the mass is an atom then that information is already encoded in the type of the particle object. In other words, mass(x::Atom) already encodes all the information, whereas atomic_mass(x::Atom) duplicates it.

Secondly, it will generalize the terminology to other particles that are not necessarily atoms.

One could introduce mass side by side with atomic_mass or it could entirely replace atomic_mass.

[mfherbst]

Personally I don't care so much about the name.

[cortner]

maybe to be part of the next breaking version discussion