Improvement: own header and source file for forest face neighbor functions

[holke]

Closes #1977

Describe your changes here:

Currently this looks like more than it acutally is, since it contains the large changes from #1736.
Merge #1736 first.

Only reordering of files, no source code added except includes.

forest.cxx is overloaded with functions.
I cleaned it up a little by moving all face neighbor related functions into 2 separate headers/source files.
One for element (i.e. same level) face neighbor and one for leaf face neighbor.

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[codecov]

Codecov Report

❌ Patch coverage is 86.24031% with 71 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.69%. Comparing base (a1a32bd) to head (df4ec94).
⚠️ Report is 19 commits behind head on main.

Files with missing lines	Patch %	Lines
...8_forest_neighbor/t8_forest_leaf_face_neighbor.cxx	83.40%	37 Missing ⚠️
src/t8_forest/t8_forest_ghost.cxx	25.80%	23 Missing ⚠️
...orest_neighbor/t8_forest_element_face_neighbor.cxx	91.17%	6 Missing ⚠️
src/t8_forest/t8_forest_private.cxx	94.44%	2 Missing ⚠️
src/t8_schemes/t8_scheme_helpers.hxx	89.47%	2 Missing ⚠️
src/t8_data/t8_containers.cxx	92.30%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1976      +/-   ##
==========================================
+ Coverage   75.66%   76.69%   +1.02%     
==========================================
  Files         105      107       +2     
  Lines       18646    18852     +206     
==========================================
+ Hits        14109    14459     +350     
+ Misses       4537     4393     -144     

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[Davknapp]

Suggested change

	Copyright (C) 2024 the developers
	Copyright (C) 2025 the developers

[Davknapp]

Suggested change

	t8_forest_element_face_neighbor (t8_forest_t forest, t8_locidx_t ltreeid, const t8_element_t *elem, t8_element_t *neigh,
	t8_forest_element_face_neighbor (const t8_forest_t forest, cosnt t8_locidx_t ltreeid, const t8_element_t *elem, t8_element_t *neigh,

[Davknapp]

Suggested change

	const t8_eclass_t neigh_eclass, int face, int *neigh_face)
	const t8_eclass_t neigh_eclass, const int face, int *neigh_face)

[Davknapp]

Suggested change

	t8_forest_element_half_face_neighbors (t8_forest_t forest, t8_locidx_t ltreeid, const t8_element_t *elem,
	t8_forest_element_half_face_neighbors (const t8_forest_t forest, const t8_locidx_t ltreeid, const t8_element_t *elem,

[Davknapp]

Suggested change

	t8_element_t *neighs[], t8_eclass_t neigh_class, int face, int num_neighs,
	t8_element_t *neighs[], const t8_eclass_t neigh_class, const int face, const int num_neighs,

[Davknapp]

Suggested change

	Copyright (C) 2024 the developers
	Copyright (C) 2025 the developers

[Davknapp]

Suggested change

	struct t8_lfn_user_data
	typedef struct

[Davknapp]

Suggested change

	};
	} t8_lfn_user_data ;

[Davknapp]

Suggested change

	struct t8_lfn_user_data *lfn_data = reinterpret_cast<struct t8_lfn_user_data *> (user_data);
	t8_lfn_user_data *lfn_data = reinterpret_cast<t8_lfn_user_data *> (user_data);

[Davknapp]

Suggested change

	struct t8_lfn_user_data user_data;
	t8_lfn_user_data user_data;