Add mortar classes

[peterrum]

Description

Add classes useful for doing mortar FEM on sliding interfaces.

Testing

A couple of unit tests. Some of them solve Poisson and Stokes equations and use multigrid.

Documentation

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Miscellaneous (will be removed when merged)

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Checklist (will be removed when merged)

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Code related list:

• All in-code documentation related to this PR is up to date (Doxygen format)
• Copyright headers are present and up to date
• Lethe documentation is up to date
• New feature has unit test(s) (preferred) or application test(s), and restart files are in the generator folder
• The branch is rebased onto master
• Changelog (CHANGELOG.md) is up to date
• Code is indented with indent-all and .prm files (examples and tests) with prm-indent

Pull request related list:

• Labels are applied
• There are at least 2 reviewers (or 1 if small feature) excluding the responsible for the merge
• If this PR closes an issue or is related to a project, it is linked in the "Projects" or "Development" section
• If the fix is temporary, an issue is opened
• The PR description is cleaned and ready for merge

[brunaccampos]

Great work! Just a couple typos and some questions

[brunaccampos]

I added some descriptions throughout the code just to help me understand what was being done. Please feel free to change anything that doesn't make sense!