Adapt Svärd-Kalisch equations to Serre-Green-Naghdi equations

[JoshuaLampert]

In this PR I try to make the implementation of the Svärd-Kalisch equations follow a more similar style as in the Serre-Green-Naghdi equations. This includes:

• having a bathymetry_type as additional field. Up to now, only a variable bathymetry is implemented (as we had before). It might be reasonable to also add the possibility to have a bathymetry_flat and then use the higher-order derivative operators for the dispersive terms (this, however, would require to have a third-derivative operator in the solver). I would like to keep this for a separate PR in the future.
• avoid any allocations in the hyperbolic part. I pre-allocated the necessary data structures in create_cache and overwrite them in rhs! now instead of always allocating them again. Now we have 0 allocations for the hyperbolic part.
• unify comments and docstrings a bit.
• reuse solve_system_matrix! from the Serre-Green-Naghdi equations.
• make some smaller fixes and simplification along the way.

Additionally, I avoided the allocations in energy_total_modified by reusing tmp1. I don't know if this is a good idea on the other hand. This seems dangerous to me since tmp1 will be changed. This also changes the returned value, right? Should I revert this change and allocate a new vector each time again, @ranocha?

[JoshuaLampert]

I plan to make a similar PR for the BBM-BBM equations in the next days, but this will be breaking because it will unify the two equation systems.

[codecov]

Codecov Report

Attention: Patch coverage is 98.07692% with 3 lines in your changes missing coverage. Please review.

Project coverage is 97.92%. Comparing base (99583ab) to head (40c19c1).
Report is 2 commits behind head on main.

Files	Patch %	Lines
src/equations/svaerd_kalisch_1d.jl	98.37%	2 Missing ⚠️
src/equations/equations.jl	94.44%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #146      +/-   ##
==========================================
+ Coverage   97.76%   97.92%   +0.15%     
==========================================
  Files          23       23              
  Lines        1747     1784      +37     
==========================================
+ Hits         1708     1747      +39     
+ Misses         39       37       -2     

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[ranocha]

Nice 👍 I have not checked all changes in detail since I assume everything should be fine when tests pass.

[ranocha]

I only have some minor suggestions for improving the documentation. From my point of view, this PR can be merged afterward. Thanks a lot!

[ranocha]

Thanks