Crash with sqw data which only has a single value of q

**Description of the error**
MDMC crash when the experimental data only includes a single value of q. MDMC needs more than one value of the experimental `q` to determine the `q_step` and therefore the shell width for the spherical averaging. To fix this issue we probably need the user to input some shell width parameter.

**Additional details**
```
File "C:\Users\...\PycharmProjects\MDMCv0.2_pilot\MDMC\trajectory_analysis\observables\fqt.py", line 284, in _calculate_Q_vectors
Q_step = (Q_values[1] - Q_values[0]) / 2.0
~~~~~~~~^^^
IndexError: index 1 is out of bounds for axis 0 with size 1
```