Purpose:returns a unique list of source names from constructed from a non-unique list of source indices Keywords: src_idx: an array of source indices sel_str: an array of source strings Returns: an array of unique source strings, the length of which will be less than or equal to the length of s Notes: --the sel_string will be returned in ascending sorted order according to their index. --used in thm_cal_fft,thm_cal_fbk --a similiar function is used to get the proper cp element any changes here should probably be mirrored there