tempo_guess_parameter check for alpha=0.0

Otherwise a not clear error results:
```
  File "./oqupy/tempo.py", line 1114, in guess_tempo_parameters
    tmp_tolerance)
  File "./oqupy/tempo.py", line 1322, in _estimate_dt_dkmax_from_bath
    cut = np.where(integrals>(1-tolerance))[0][0]
```