Use consistent nomenclature for ifo_id / pad_id / enabled_ifo_id / cur_ifo / iifo / i / j

Throughout the code we use a multitude of variable names when referring to either the n'th enabled IFO or the n'th possible IFO. This leads to hard to detect bugs such as #81 (closed) #88 (closed) We should decide on a consistent name for these ids.