Resolve "Add scaling field input"

What does this MR do?

Add option to input global Miller scaling fields and supply framework for generalized scaling field input

• Revamp interpolators to be more generic and hold actual data

  • Add base class and factory

  • Add Nearest-Neighbor interpolator

  • [ ] Have interpolators implement GridFunctionBase

    Not possible because current interface does not supply the necessary grid view.

  • Add unit test for interpolator

• Add adapters for evaluating scaling factors

  • Add base class and factory

  • Add dummy scaling adapter

  • Add Miller scaling adapter

  • [ ] Have scaling adapters implement GridFunctionBase

    Not possible because current interface does not supply the necessary grid view.

  • Add unit test for scaling adapter

• Add infrastructure

  • Add section to documentation of soil parameters and architecture
  • Add ExponentialConverter with parameters to field-parameters.xml
  • Add parameterization file param.yml to default_files

• Rewrite test-parameterization not to use RichardsSimulation anymore.

• Add unit test test-parameterization-scaled, ensuring that scaling is correctly applied wrt. the regular parameterization.

Is there something that needs to be double checked?

• test-parameterization does not compile a RichardsSimulation anymore. Therefore, many source files are not compiled with --coverage flags and don't appear in the gcovr output at all. This artificially raises the Coverage. Not sure how to solve this.

Can this MR be accepted?

• Implemented stuff
• Added unit tests
• [ ] Added system test Not required. test-parameterization-scaled checks if scaling can be applied in context of FlowParameters.
• Merged master into this branch
• Pipeline passing
• Added entry to CHANGELOG.md

Related issues

Closes #110 (closed), #18 (closed)