Fix initial condition from Dingemans experiment for BBM-BBM equations

[JoshuaLampert]

The results from the BBM-BBM equations for the experimental setup was missing a vertical translation of the water surface and the bathymetry. This is fixed here and the notation about the bathymetry $b$ and still water depth $D$ is used consistently.
We have $D = \eta_0 - b$ for the still water surface $\eta_0$. The still water surface can be arbitrary in the case of the Svärd-Kalisch equations, but needs to be 0 for the BBM-BBM equations (otherwise there would be an additional term in the equations).

[codecov]

Codecov Report

Attention: Patch coverage is 96.55172% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 96.17%. Comparing base (f206ad8) to head (0fb139a).

Files	Patch %	Lines
src/equations/bbm_bbm_1d.jl	85.71%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #91      +/-   ##
==========================================
+ Coverage   96.15%   96.17%   +0.02%     
==========================================
  Files          17       17              
  Lines        1065     1073       +8     
==========================================
+ Hits         1024     1032       +8     
  Misses         41       41              

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[ranocha]

Thanks!