Share Atom Types between species nodes.

[rprospero]

Describe the feature / issue
Atom types defined in one species should be possibly shared with other nodes.

Additional details
A species node should both output a vector of atom types and allow for an optional input vector of atom types. When the input vector is updated, all atom types of the same name are overwritten (this is the only safe way to keep the atom types up to date with their source) and missing atom types are appended.