Construct quantum-dots with more than one type of ligand

[BvB93]

• Construct quantum-dots with more than one type of ligand.
• Initially these structures will not be supported for the Nano-CAT woorkflows.
  This might be changed at a later date.

[BvB93]

Proposed syntax:

optional:
    core:
        symbol: 'Cl'

    qd:
        multi_ligand:
            f: [0.3, 0.2, 0.1, 0.05]
            mode: 'uniform'
            ligands:
                - 'OCCCCCCCCCC'
                - 'OCCCCCCC'
                - 'OCCCC'

and / or

optional:
    core:
        symbol: 'Cl'

    qd:
        multi_ligand:
            dummy: ['F', 'Br', 'I']
            ligands:
                - 'OCCCCCCCCCC'
                - 'OCCCCCCC'
                - 'OCCCC'

[BvB93]

Implementation of the dummy-based approach is progressing nicely, the subset-based approach will likelly be implemented the patch afterwards.

[BvB93]

Implemented the dummy-based approach in #92.