add classes and methods for remaining ogs6_prj main tags

• Check all benchmarks for additional main tags in the respective *.prj files.
  • Double check with OGS6 documentation to include all options
• Add classes and methods

changed due date to December 11, 2020

changed milestone to %Core Functionality

added feature label

List of ogs6 *.prj file main tags to implement (based on ogs6 docs)

• chemical_system
• curves
• geometry
• insitu
• linear_solvers
• local_coordinate_system
• media
• mesh
• meshes
• nonlinear_solvers
• parameters
• process_variables
• processes
• python_script
• search_length_algorithm
• test_definition
• time_loop

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