Replace omp_set_nested (deprecated) with omp_set_max_active_levels

[servantftransperfect]

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omp_set_nested is replaced by omp_set_max_active_levels

omp_set_nested was a binary choice (0 to desactivate nested omp loops, 1 to activate). omp_set_max_active_levels set the number of allowed nested loops (1 to desactivate, n!=1 to set the exact number of activated nested loops).

I assumed in the code we only needed 1 nested loop, so i set omp_set_max_active_levels to 2 when needed.