onlyXYbasePairing: repulsive part for non-XY

Instead of completely disabling base pairing between non-XY pairs with
onlyXYbasePairing, we now keep the original repulsive part in-place. This
means that we can disable onlyXYbasePairing at any time without worrying
about base pairs flying apart that came too close (because the default
exclusion works on smaller distances than the repulsive part of the base
pairing interaction).

+ Changed offset in calcVbasePair so it's zero at infinity. This does not
change userspace, as we always returned the rescaled value compared to the
truncation length.