Fix SEI example notebook

[brosaplanella]

Fix the failing notebook, as identified in #3110 (comment). I also wrote some extra details about how to fix the moving boundary, which were part of the non-dimensionalisation and thus were lost with the shift to dimensional models.

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[brosaplanella]

@arjxn-py this should fix the problems with #3110, so once we merge this into develop you can merge develop back into your branch.

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (f1f3fe5) 99.71% compared to head (7c86beb) 99.71%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #3166   +/-   ##
========================================
  Coverage    99.71%   99.71%           
========================================
  Files          248      248           
  Lines        18751    18751           
========================================
  Hits         18697    18697           
  Misses          54       54           

Impacted Files	Coverage Δ
pybamm/parameters/lithium_ion_parameters.py	100.00% <ø> (ø)

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[arjxn-py]

@arjxn-py this should fix the problems with #3110, so once we merge this into develop you can merge develop back into your branch.

Thanks so much @brosaplanella 😄, will merge my branch with it.

[rtimms]

thanks @brosaplanella looks good to me!