Clean up t8_cmesh.c

[chiaraMaHe]

ToDo's:

• Document file
• Is_checking is not thread safe. If two threads call cmesh routines that call t8_cmesh_is_committed, only one of them will correctly check the cmesh
• 108 - check more conditions that must always hold after commit
• t8_cmesh_get_tree should get a gloidx?
• hide t8_cmesh_get_first_tree
• hide t8_cmesh_get_next_tree
• 434 - move check to commit
• 434 - If cmesh is partitioned and part has no trees then the dimension remains unset forever
• Rewrite t8_cmesh_is_equal
• Bug 682: running with two processes and a cmesh of one T8_ECLASS_LINE, then on both processes the face_neighbors and vertices arrays of the single tree point to the same physical memory. (face_neighbors on both processes are equal and vertices on both processes are equal)
• t8_cmesh_bcast send the tree’s vertices
• t8_cmesh_bcast rewrite
• 703 - would it be useful to allow bcast even if the cmesh is referenced? But then the bcasted version on other procs would have a different refcount, than the cmesh on the root
• 735 - use BIcast
• 1104 – check whether this face is a boundary face
• 1174 – implement
• rewrite t8_cmesh_new_translate_vertices_to_attributes