Rename the keyword gravity_constant to gravity for the ShallowWaterEquations3D

[patrickersing]

This PR renames the gravity_constant keyword in the equations constructor to gravity and fixes the docstrings/comments to describe this quantity as the gravitational acceleration instead of the gravitational constant. This will make it consistent with the actual field equations.gravity and avoid any confusion between gravitational acceleration and the gravitational constant. See trixi-framework/TrixiShallowWater.jl#82, trixi-framework/TrixiShallowWater.jl#84, trixi-framework/Trixi.jl#2357

[tristanmontoya]

LGTM!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.39%. Comparing base (f250b56) to head (7bb5be1).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #81   +/-   ##
=======================================
  Coverage   90.39%   90.39%           
=======================================
  Files          22       22           
  Lines        2104     2104           
=======================================
  Hits         1902     1902           
  Misses        202      202           

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