Clean up t8_cmesh_examples.c

[chiaraMaHe]

ToDo's:

• 33 - In p4est a tree edge is joined with itself to denote a domain boundary. Will we do it the same in t8code? This is not yet decided, however the function below stores these neighbourhood information in the cmesh.
• 36 - Eventually we may directly partition the mesh here Offset-1 is added to each tree_id, this is used in i.e. t8_cmesh_new_disjoint_bricks, if offset is nonzero, then set_partition must be true and the cmesh is partitioned and has all trees in conn as local trees. The offsets on the different processes must add up!
• 77 - Check offsets for consistency
• 107 – code duplication – make it a function
• t8_cmesh_new_hypercube - upgrade with int x,y,z for periodic faces
• t8_cmesh_new_prism_cake - seems to be a lot of memory, can we also get by with only 6 * 3 doubles?