Clean up t8_cmesh_commit.c

[chiaraMaHe]

ToDo's:

• Document t8_cmesh_commit.c
• 163 – attribute array has to be sorted (by now) – find another solution
• 229 – reset cmesh
• 233 – is this shared array needed?
• 317 - optimize if non-hybrid mesh
• 388 – find better solution for if/else if
• 458 - attribute is required to be sorted right now. Think this over
• t8_cmesh_commit – set boundary face connection
• t8_cmesh_commit - Implement a debug check for mesh consistency between processes
• t8_cmesh_commit - split this up into smaller functions
• 627/668 – remove code duplication