docs: def set_instrument(self, instrument=None):
          """Set the instrument (i.e. tell cellpy the file-type you use).
  
          Args:
              instrument: (str) in ["arbin", "bio-logic-csv", "bio-logic-bin",...]
  
          Sets the instrument used for obtaining the data (i.e. sets fileformat)
  
          """
          if instrument is None:
              instrument = self.tester
  
          if instrument in ["arbin", "arbin_res"]:
              self._set_arbin()
              self.tester = "arbin"
  
          elif instrument == "arbin_sql":
              self._set_arbin_sql()
              self.tester = "arbin"
  
          elif instrument == "arbin_experimental":
              self._set_arbin_experimental()
              self.tester = "arbin"
  
          elif instrument in ["pec", "pec_csv"]:
              self._set_pec()
              self.tester = "pec"
  
          elif instrument in ["biologics", "biologics_mpr"]:
              self._set_biologic()
              self.tester = "biologic"
  
          elif instrument == "custom":
              self._set_custom()
              self.tester = "custom"
  
          else:
              raise Exception(f"option does not exist: '{instrument}'")