Compute sediment calibration before interpolation can crash

If we try to compute sediment calibration before interpolation (or click plot sample button), the program can crash.

Traceback (most recent call last):
  File "/home/parouby/INRAE/acoused/View/sediment_calibration_tab.py", line 1624, in function_pushbutton_compute_calibration
    self.compute_ks()
  File "/home/parouby/INRAE/acoused/View/sediment_calibration_tab.py", line 1637, in compute_ks
    num_sample=stg.sand_sample_target[0][1], r_grain=stg.radius_grain_sand,
IndexError: list index out of range