Extraction of coherent pairs in `InteractionPointGuineaPig` is actually extracting number of incoherent pairs

[ben-c-2013]

Event.num_coherent_pairs() returns Event.num_pairs. Currently, for InteractionPointGuineaPig, this is extracted from the GUINEA-PIG output file by parsing the line containing 'n_pairs =' (see guineapig_wrapper.py ). This is the number of produced incoherent pairs.