DataObjects should support multiple COs

[jvita]

This may be able to be postponed for now, however I can definitely imagine use-cases where it would be necessary:

• PI = migration barrier, COs = NEB images
• PI = elastic constants, COs = strained configurations
• PI = energy difference, COs = start/end configurations (note: although the *-energy PDs allow for reference energies, it may be important to be able to explicitly state the corresponding reference configuration)