Add typedefs for return values

[sandro-elsweijer]

We never defined what our return values mean and how they should look like.
Maybe we can add
typedef t8_success 1
typedef t8_failure 0
so that the return values of functions are defined and understandable

[sandro-elsweijer]

Maybe we can take a look at how sc does it

[sandro-elsweijer]

Here is an example of two functions returning the exact opposite on success:

t8code/src/t8_vtk/t8_vtk_write_ASCII.cxx

Line 561 in af71795

t8_forest_vtk_write_cells (t8_forest_t forest, FILE *vtufile, const int write_treeid, const int write_mpirank,

t8code/src/t8_vtk.h

Line 81 in af71795

t8_write_pvtu (const char *filename, int num_procs, int write_tree, int write_rank, int write_level, int write_id,