Refactoring wave solver: new folder organization

[acitrain]

This PR is the final part of the refactoring for wave solvers

It mainly contains new folder organisation + small cleanup

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 53.19%. Comparing base (9289b76) to head (17b1551).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #3074   +/-   ##
========================================
  Coverage    53.19%   53.19%           
========================================
  Files          989      989           
  Lines        83646    83646           
========================================
  Hits         44498    44498           
  Misses       39148    39148           

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