Mortar: fix mapping at multigrid levels

[brunaccampos]

Description

The same mapping object was being used at every multigrid level which, when the mortar feature is enabled, led to incorrect setup of mapping within level operators. This PR fixes this, as well as some other minor typos/descriptions.

Solution

A MappingQCache object is now created at every level when set enable = true in the mortar subsection.

Testing

No new tests were added. This issue was only spotted in larger simulations, which is why existing application tests were not affected.

Documentation

Miscellaneous (will be removed when merged)

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
• Fix 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:

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• If the fix is temporary, an issue is opened
• The PR description is cleaned and ready for merge