Rename all relevant matrix-free files using Fluid Dynamics prefix

[lpsaavedra]

Description

This PR changes the name of the main class and the name of the files related to the matrix-free application. Is the first PR on a series of PRs that aim to rename all the Navier Stokes solvers of Lethe using the prefix FluidDynamics.

Testing

All the matrix-free tests should pass as only the name of the class and the name of files were changed.

Checklist (will be removed when merged)

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Code related list:

• The branch is rebased onto master
• Code is indented with indent-all and .prm files (examples and tests) with prm-indent
• Changelog (CHANGELOG.md) is up to date if the refactor affects the user experience or the codebase

Pull request related list:

• No other PR is open related to this refactoring
• Labels are applied
• There are at least 2 reviewers (or 1 if small feature) excluding the responsible for the merge
• The PR description is cleaned and ready for merge