Enable high order for fwi in acoustic wave solver

[acitrain]

This PR generalize the formula for gradient computation inside the acoustic second order wave solver to high orders

The main change is the computation of the number of nodes per elements instead of hardcoded it to 8

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 10 lines in your changes missing coverage. Please review.

Project coverage is 53.16%. Comparing base (cb20b88) to head (06a595e). Report is 323 commits behind head on develop.

Files with missing lines	Patch %	Lines
...olvers/wavePropagation/AcousticWaveEquationSEM.cpp	0.00%	10 Missing :warning:

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2998      +/-   ##
===========================================
- Coverage    53.16%   53.16%   -0.01%     
===========================================
  Files          981      981              
  Lines        83492    83494       +2     
===========================================
  Hits         44386    44386              
- Misses       39106    39108       +2     

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[rrsettgast]

@sframba Is this deprecated or superseded?

[acitrain]

We can close it, in fact the formula was right at the beginning so no need to change it