Error N1 != N2 assert in basic tutorial

[djgroen]

@eloisabentivegna reported this error during our 24-hour Hackathon.

[eloisabentivegna]

The error occurs at:

https://github.com/UCL-CCS/EasyVVUQ/blame/e455582c290eca1df5d7fa7a8044865ab8d69836/tutorials/basic_tutorial.ipynb#L663

The tutorial works fine up to this point, but this cell returns:

  File "/opt/miniconda3/envs/sv/lib/python3.11/site-packages/easyvvuq/analysis/pce_analysis.py", line 495, in analyse
    dY_hat = build_surrogate_der(fit, verbose=False)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/miniconda3/envs/sv/lib/python3.11/site-packages/easyvvuq/analysis/pce_analysis.py", line 335, in build_surrogate_der
    assert(n1 == n2)
           ^^^^^^^^
AssertionError

The subsequent cells execute with no errors, and match the output of the notebook on the repository. I am on the dev branch.