Implementation of the acoustic wave problem

[GiovanniCanali]

Description

This PR fixes #716

Checklist

• Code follows the project’s Code Style Guidelines
• Tests have been added or updated
• Documentation has been updated if necessary
• Pull request is linked to an open issue

[GiovanniCanali]

Added minor changes to the doc of all problems in the zoo, namely default values, explicit argument specifications, and missing doc strings.

[dario-coscia]

Thanks for the PR. I added few comments.

1. For the wave equation everything is fine, just need to add the mathematical description.
2. There are a bunch of other files which should be removed from this PR. Also many file introduce value=0 instead of 0. I would not do it, motivation below. For the fact that you change also the doc in many files to specify the init variables, what is the rationale? They are already written in the doc.