BUG: EDF 2655: Low performance of hexa to tetra splitting

BUG: EDF 2655: Low performance of hexa to tetra splitting

The whole procedure performance was almost O(n^2) due to insertion of for example 5 elements in a mesh with a free ID at the beginning. The second element is then inserted with a O(n) complexity.
The hexas are now removed after all tetra insertions, which guarantees a O(n) complexity for the whole procedure at a limited memory cost (transient additional cost of 1/5 of total memory occupation at the end).