text: Title: Unnecessary arguments in `core/scattering1d/scattering1d`
  Body: Not a big priority, but an easy simplification. The current prototype of `scattering` is: (phew!)
  
  ```python
  scattering1d(x, pad, unpad, backend, log2_T, psi1, psi2, phi, pad_left=0,
          pad_right=0, ind_start=None, ind_end=None, oversampling=0,
          max_order=2, average=True, vectorize=False, out_type='array')
  ```
  
  This didn't come to my attention while reviewing #673 , but the 5th input parameter, `log2_T`, is unnecessary: it is already available as `phi["j"]`. So we could just remove it, and put `log2_T = phi["j"]` at the top of the function.
  
  Thoughts?
  
  