Type error in ```docs/notebooks/01-using-solvers.ipynb```

[meyer-nils]

During testing for the review (openjournals/joss-reviews#9733), I noticed a TypeError in one of the notebooks:

     20 def custom_step_function(c, t):
---> [21](vscode-notebook-cell:?execution_count=17&line=22)     c = torch.clip(c, 0, 1)
     22     mu_hom = 18 / eps * c * (1 - c) * (1 - 2 * c)
     23     c_hat = fft.fftn(c)

TypeError: clip() received an invalid combination of arguments - got (int, int, int), but expected one of:
 * (Tensor input, Tensor min = None, Tensor max = None, *, Tensor out = None)
 * (Tensor input, Number min = None, Number max = None, *, Tensor out = None)

Looking at TimeDependentSolver.solve(), this seems to be caused by a mismatch of the call signatures for the step function. Swapping c and t in the custom step function resolves the issue.

[meyer-nils]

One more minor note regarding this notebook: Could there be a typo in the chemical potential $\mu=\delta f_\text{int}/\delta\psi$ confusing $\psi$ and $\phi$?

[daubners]

sorry my bad. between the submission in July and now, I've updated the scheme for ODEs to the more common (t, u) convention. All the notebooks have been updated in #41