Fix initialization of SolutionSavingCallback

[efaulhaber]

Without the reset of latest_saved_iter, the SolutionSavingCallback can't be reused for a second simulation. Defining the callback in the REPL and passing it to trixi_include fails on main when running a second simulation with the same callback.

Also, there is no reason not to make this struct mutable, so we can get rid of the awkward "vector of size 1" approach.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 70.82%. Comparing base (fc108d6) to head (c3f2154).

Files	Patch %	Lines
src/callbacks/solution_saving.jl	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #456      +/-   ##
==========================================
- Coverage   70.84%   70.82%   -0.02%     
==========================================
  Files          66       66              
  Lines        3807     3808       +1     
==========================================
  Hits         2697     2697              
- Misses       1110     1111       +1     

Flag	Coverage Δ
unit	70.82% <0.00%> (-0.02%)	⬇️

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