Introduce sparse free-slip BC

Squashed commit of the following:

commit 8f68fe66
Author: Frederik Hennig <frederik.hennig@fau.de>
Date:   Wed Feb 5 09:11:41 2025 +0100

    user guide on sparse free-slip. Fix CI config.

commit 12935ac4
Author: Frederik Hennig <frederik.hennig@fau.de>
Date:   Tue Feb 4 16:52:26 2025 +0100

    Add Free-Slip Pipe Test case.

commit ed6f686c
Author: Frederik Hennig <frederik.hennig@fau.de>
Date:   Tue Feb 4 13:13:15 2025 +0100

    fix freeslip indexing bug. test suite WIP.

commit ec4f2e93
Author: Frederik Hennig <frederik.hennig@fau.de>
Date:   Mon Feb 3 16:39:51 2025 +0100

    freeslip is still irregularily segfaulting. I dont know why.

commit f5b4f809
Author: Frederik Hennig <frederik.hennig@fau.de>
Date:   Mon Feb 3 15:14:43 2025 +0100

    irregular free-slip implementation