Resolve "Functionality for generating structures with a specified cluster vector"

With this MR it becomes possible to generate SQS or any other structure that should closely resemble a specific target cluster vector.

Design

• The user interface is defined in icet/tools/structure_generation.py
• A special ensemble is defined in mchammer/ensembles/structure_annealing.py. This ensemble holds simultaneously multiple supercells and one calculator for each supercell. The reason is that the structure search is not performed only within one supercell shape, but among several. A trial step therefore consists of randomly choosing a supercell before carrying out a trial step.
• The target function is calculated with a new calculator, mchammer/calculators/target_vector_calculator.py
• The new functions are unit tested and their docstrings are included in the documentation
• A new tutorial ("Special quasirandom structures") has been added
• The MR also involves a convenience function for enumerating supercell shapes, which may also be useful outside the scope of #257 (closed)

Closes #257 (closed)