Bug: Empty process offset computation in t8_cmesh_gather_treecount_ext is wrong in some cases

[holke]

When constructing a cmesh.
If t8_cmesh_load is used and the current process is empty, then the cmesh sets its num_local_tree to 0, its first_tree to -1 and first_tree_shared to 0.

This can break the offset computation for empty processes in t8_cmesh_gather_treecount_ext, because t8_offset_next_nonempty_rank may identify an empty rank as nonempty in the case that multiple empty are in a row.

Note, that this behaviour may also happen in potential new cmesh construction methods (that do not exist yet, but use a similar logic to set the empty processes).

Here is how we can fix the behaviour:

0. Write a test case that covers this beahviour and currently fails (i.e. load (or simulatie a load) a cmesh such that multiple empty processes are in a row).
1. Do not use an offset value to indicate empty processes in t8_cmesh_gather_treecount_ext. Instead use a second array of bool/int with values 0 and 1 for empty or non-empty processes and Allgather that array as well.
2. In the empty correction starting from

   t8code/src/t8_cmesh/t8_cmesh_partition.cxx

   Line 214 in e9a123b

   if (has_empty) {

   use the new array to determine the next non-empty process instead of the function t8_offset_next_nonempty_rank
3. Make the execution of this empty correction optional controlled by a function parameter, which set true by cmesh_load but set false by other methods (i.e. uniform_bounds)
4. Outsource the code starting from

   t8code/src/t8_cmesh/t8_cmesh_partition.cxx

   Line 214 in e9a123b

   if (has_empty) {

   in an own function t8_cmesh_offset_empty_correction

[holke]

@Davknapp @lukasdreyer ping

[lukasdreyer]

@Davknapp: I think there is a similar problem, for t8_forest_partition_create_tree_offsets after calling t8_forest_partition_given, when you have 2 empty procs next to each other.

In t8_forest_partition_given, the following lines set first and last local tree in a way, that does not fit the offset rule needed to compute num_local_trees correctly at a different place. Therefore, offset is computed to be 0, and not the treeid of the next tree.

This bug can be seen by executing
mpirun -n 8 ./test/t8_forest/t8_gtest_forest_commit --gtest_filter=*prism_cake*