Examples: tweaked userapi free surface notebook to use MPI-safe implementation

[EdCaunt]

Updated this notebook with a better free surface implementation in the vein of Mathias' implementation in the acoustic examples.

[review-notebook-app[bot]]

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[codecov[bot]]

Codecov Report

Merging #1952 (ad025fd) into master (9061a67) will increase coverage by 0.13%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1952      +/-   ##
==========================================
+ Coverage   87.65%   87.79%   +0.13%     
==========================================
  Files         218      221       +3     
  Lines       38260    38642     +382     
  Branches     4987     5000      +13     
==========================================
+ Hits        33538    33926     +388     
+ Misses       4168     4165       -3     
+ Partials      554      551       -3     

see 6 files with indirect coverage changes

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