roughness operator (#175)

# Espresso Pull Request Template

## Description

Please provide a clear and concise description of the changes you've
made.

If you are adding a new `EspressoProblem`, read the 
[Contributor's
Guide](https://geo-espresso.readthedocs.io/en/latest/contributor_guide/index.html)
as a reference. Otherwise refer to the 
[Developer's
Guide](https://geo-espresso.readthedocs.io/en/latest/developer_guide/develop.html).

## Type of Change

Please indicate the type of change this pull request represents:

- [ ] New Espresso problem or example
- [ ] Espresso core
- [ ] Espresso machine
- [ ] Documentation
- [x] Others (please specify here)

## Checklist for new `EspressoProblem` (remove me if not applicable)

### Requirements

Please list any requirements or dependencies for running the problem
here.

### Additional information

Provide any additional information on how to run the problem or context
about it here.

### Jupyter Notebook

If applicable, provide a brief description of the Jupyter notebook
included with this problem.

### Testing

To validate your problem:

```console
$ python espresso_machine/build_package/build.py --pre --post -c <example_name>
```

Please confirm that you have tested your problem:

- [x] I have checked that all tests pass locally with my changes
- [x] I have ensured that my code follows the style guidelines of this
project

### Licence

Confirm the licence under which this problem is distributed.

## Checklist for Espresso core (remove me if not applicable)

- [ ] My changes are backward compatible
- [ ] The new contribution generation script is compatible with the new
changes
- [ ] The build script is compatible with the new changes
- [ ] The validation script is compatible with the new changes
- [ ] The documentation is up to date with the new changes
- [ ] I have formatted the Python files with `black`

## Checklist for Espresso machine (remove me if not applicable)

- [ ] My changes are backward compatible
- [ ] The documentation is up to date with the new changes
- [ ] I have formatted the Python files with `black`

## Checklist for Espresso documentation (remove me if not applicable)

- [ ] I have built the documentation by running `make html` in the
`docs/` folder
- [ ] I have served the website locally by running `python -m
http.server 8000 -d build/html` and checked the changes at
`localhost:8000`

## Additional Information

Please provide any additional information or context about the pull
request here.