[App|PVTU2VTU] Accelerate tool pvtu2vtu

The OctTree changes where moved to a separate MR

Please review the OctTree changes first. After OctTree changes are merged I'll rebase this MR (which will reduce the changes in this MR).

1. Feature description was added to the changelog
2. Tests covering your feature were added? - reference file was updated
3. Any new feature or behaviour change was documented? - no new feature implemented

Run time tests

• unit cube (1e6 hexes) partitioned into 3 sub-domains
• used hyperfine for run time statistics

Current master

Benchmark 1: pvtu2vtu -i cube_1e6_ts_1_t_1_000000.pvtu -o x.vtu
  Time (mean ± σ):     1369.399 s ± 12.500 s    [User: 1362.585 s, System: 0.722 s]
  Range (min … max):   1356.106 s … 1397.235 s    10 runs

This MR

Benchmark 1: pvtu2vtu -i cube_1e6_ts_1_t_1_000000.pvtu -o x.vtu
  Time (mean ± σ):      6.904 s ±  0.240 s    [User: 6.015 s, System: 0.729 s]
  Range (min … max):    6.574 s …  7.214 s    10 runs