Resolve "Add example that demonstrates parallel WL"

Description

This MR introduces an advanced example of how to perform parallel Wang-Landau simulations as well as analyse the data thus obtained. The user guide has also been updated so that it encompasses this example.

Comments

To ensure that the time required for running the example does not exceed 15 minutes, both the fill factor and flatness limits as well as the system size have had to be reduced. Even so, the diagrams produced from the output, which can be found in the user guide, are comparable to those obtained for larger supercell sizes and stricter convergence criteria. The example is, therefore, deemed to serve its intended purpose.

Summary of changes

• The script for running parallel WL simulations has been updated.
• A script has been added that can be used to analyse the output from the simulations.
• The relevant sections in the user guide have been rewritten, which includes adding new versions of the related figures.
• It has been ensured that all WL examples are run in the GitLab CI

Closes #436 (closed)