Optimized perp. coupling calculation

and corrected non-relevant error in getPerpCoupling in subeuo.cpp
Introduced calculation intervall for RKKY coupling

The coupling is now only calculated every 'max3' times.
If da1<tol, max3 is decreased by a factor 1/2 until it
is equal to 1. Hereby the coupling is calculated in each iteration
step at the end of the iteration procedure.