get cosmic string exclusion plots for large-loop models

The codes obtain the exclusion plots from the efficiency curves, for the large loop models considered in the O1 & O2 search. The lalapps_string_cs_gamma_largeloops.py can obtain the detection rate as a function of Gmu and p, for three models considered in the O1 and O2 searches. A function is added in cs_gamma.c as an ingredient to calculate this rate. Given the output and the live time of the search, lalapps_string_contour_plotter_largeloops.py obtains the final exclusion plot.