fix plotting 1d function

[JoshuaLampert]

We make one dummy call to func to obtain the number of variables. We should pass a one-entry SVector for x instead of a scalar since we also pass SVectors later when getting the plotting data. This is to be consistent when calling initial conditions, where we also always pass the spatial variable as a vector (in 1D a vector with one element). Some initial conditions from Trixi.jl are compatible to work with both real values and vectors, but not all. Therefore, passing only a real value will not work with all predefined initial conditions.

[github-actions]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.88%. Comparing base (ba30bb3) to head (aa92fe7).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2271   +/-   ##
=======================================
  Coverage   96.88%   96.88%           
=======================================
  Files         490      490           
  Lines       39491    39491           
=======================================
  Hits        38257    38257           
  Misses       1234     1234           

Flag	Coverage Δ
unittests	96.88% <100.00%> (ø)

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