Further improved the summation of the fermionic contribution for an L^4 lattice. It also uses that
the components p_mu=(pi+x)L/ and p_mu=(pi-x)/L give equal contributions.

Factor 15

also added a file to show the principle of the summation

The same thing is possible for the propagatorsums, but it's tedious to code up.
added the resttriction of the momentumsummation also to the propagators (only for L^4 and only for lowmem)

lowmem is now faster than without. Have to revisit the thing at one point