Resolve "Concentration restricted structure enumeration"

Description

This MR adds a simple concentration restricted enumeration functionality to icet (#202 (closed)).

Comments

It works in a fairly naive way: all "labelings" for a given cell size are generated, but those with the wrong concentrations are thrown away. This is done at a quite early stage in the algorithm, so it is not as inefficient as it may sound, but I doubt this implementation will work for clathrates for example.

Summary of changes

• A few changes in ./icet/tools/structure_enumeration.py and a new module ./icet/tools/structure_enumeration_support/labeling_generation.py
• Tests added in ./tests/integration/tools/test_structure_enumeration.py and ./tests/unittest/tools/test_labeling_generation.py
• Example added in ./examples/enumerate_structures.py
• Example described in ./doc/userguide/source/advanced/enumerate_structures.rst

Closes #202 (closed)