Fix omp performance issue using a manual condition

Using a manual cut off prevent omp overhead. I run it on codespeed and we have the 75% faster for arc_dist but we don't have the 350% slower for wave_simulation (there is no speed down indeed).

The only perf issues are :
fibo : 25% slowdown (well, it doesn't use numpy so it is certainly noise)
wdist : 25% slowdown too (I have no explaination for this)