Ability to assign fractional masses to SpeciesAtoms

[trisyoungs]

Describe the feature / issue
To achieve better control over phantom atoms, as well as allow the use of e.g. core-shell models, we need the ability to be able to specify a non-standard mass for individual SpeciesAtoms, rather than rely on those tabulated values in AtomicMass.

This probably warrants the introduction of a new type of phantom atom which can contribute its mass to the overall mass of its Species.