[WIP] Oceanic extension cookbook

[valemagni]

Add cookbook to illustrate the parameters and processes controlling non-linear deformation of oceanic lithosphere. This is the first parameter file (cookbook/oceanic_extension/oceanic_extension.prm) of a series of cookbooks that systematically build in complexity, and thus serves as a reference model for the remaining cookbooks. Still work in progress.

Pull Request Checklist. Please read and check each box with an X. Delete any part not applicable. Ask on the forum if you need help with any step.

Describe what you did in this PR and why you did it.

Before your first pull request:

• [x ] I have read the guidelines in our CONTRIBUTING.md document.

For all pull requests:

• [x ] I have followed the instructions for indenting my code.

[anne-glerum]

Thank you Valentina!

[naliboff]

@valemagni - Thanks for going through all these changes! At some point it would be good to squash many of these commits. Perhaps it could be reduced down to the initial commit and a second commit "address comments on parameter file"?

[naliboff]

@valemagni - Also, it looks like the tester for indent+documentation is not passing. Can you run "make indent" and push the commit? Thanks!