EXPERIMENTAL change foward/backward to centered ds
EXPERIMENTAL change foward/backward to centered ds

Hopefully this will significantly lower the parallel leakage term
at the presumable cost of stability

mwiesenberger/feltor,feltor-dev/feltor,mwiesenberger/feltor,mwiesenberger/feltor,feltor-dev/feltor,mwiesenberger/feltor,feltor-dev/feltor,feltor-dev/feltor