Add Selke 2D liquid flow notebook

Created a new Liquid Flow Benchmark for the Selke 2D process with Dirichlet boundary conditions showing the complete workflow with OGSTools:

• Loading meshes and project files
• Building model, visualizing boundary conditions
• Running simulation
• Rescaling result and show final state
• Probing at points and plotting over time
• Modifiying project file, running again and visualizing differences at simulation end

1. Tests covering your feature were added?
2. Any new feature or behaviour change was documented?

• Add this merge request to the changelog