Missing simple repeated molecule combination function

[ajkerr0]

For repeated combination of molecules, there needs to be a simple function to call that takes the base molecule, list of indices, and list of attachments (indexed the same as the previous list). It would be nice to not have to build Calculation objects all the time to do this. Ideally this new function would be implemented in kappa.Calculation. We have a few choices:

1. alter _combine function to take single arguments or lists of arguments
2. write a new function that calls _combine repeatedly
3. some other way I forgot.