Added a speciallization for the computation of ther fermionic contribution in the case of a L^4 lattice

First test suggest an improvement in speed by a factor of 6..8 (if the bosonic contribution is the dominant part)

however, the results were a little bit different, but this might be related to rounding in adding up a lot of small doubles

Next step (before further improvements are made) will be to sum up the fermionic part in steps to reduce the loss of accuracy