docs: def get_residue_mapping(self):
          """this function maps the chain and res ids "A 234" to values from [1-N]"""
  
          resid_list = self.aa_resids()
          # resid_set  = set(resid_list)
          # resid_lst1 = list(resid_set)
          # resid_lst1.sort()
          map_res_id = {}
  
          x = 1
          for resid in resid_list:
              # map_res_id[ int(resid[1:].strip()) ] = x
              map_res_id[ resid ] = x
              x+=1
          return map_res_id