Add plot_analytical to plot recipes

[JoshuaLampert]

Before, plot_initial was misleading because it plotted the analytical solution if available. This adds a keyword to plot the analytical solution and lets plot_initial indeed plot the initial condition. See also NumericalMathematics/DispersiveShallowWater.jl#277.

[codecov]

Codecov Report

❌ Patch coverage is 95.45455% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.47%. Comparing base (3fcfdab) to head (a98a0f4).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/visualization.jl	95.45%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #72      +/-   ##
==========================================
+ Coverage   98.45%   98.47%   +0.02%     
==========================================
  Files          24       24              
  Lines        1807     1833      +26     
==========================================
+ Hits         1779     1805      +26     
  Misses         28       28              

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