Fix speed issue for pfold on fast periods

Felix found an issue for fast periods (relative to GTI windows or LC
bins) where a huge performace penalty occured because we were looping
through each covered period one by one. For slow rotations/short
windows this is perfectly fine (and fast). To cover the case of fast
periods I changed the loop to go through all time frames of one phase
bin. Currently the change is applied if the time window covers more
than 5 phases. Might need some tweaking.