Added make_arf2.pro and accompanying files. 

This code computes the diagonal part of the response matrix. It uses the balloon altitude and the source elevation to compute the atmospheric transmission. It also takes an offset angle (in arc seconds) and computes the offset effective area of the optics. The keyword summed_eff_area, if set, sums the effective areas for all 8 modules. If this is not set, then the nshells input must be set to either 13 or 14. This diagonal part of the response then needs to be combined with heroes_8detectors.rmf (the detector response which includes response to single events, measured ratio from Xenon escape peaks to Am and Tb, and detector resolution, summed over all detectors.) read_rmf.pro reads this file.