SmallDeformation uses media for solid density evaluation

Use MPL for solid density in the SmallDeformation process.

This is breaking change; usage of <solid_density> tag in the process' description is no longer valid. A corresponding error message is printed.

All the files were changed automatically using the following python-script convert.py

1. Feature description was added to the changelog
2. Tests covering your feature were added?
3. Any new feature or behavior change was documented?