Enable save solution with time intervals for SimpleIntegratorSSP

[amrueda]

This PR implements some modifications to SimpleIntegratorSSP, such that SaveSolutionCallback can be used with time intervals (keyword dt) instead of time-step intervals (keyword interval).

[github-actions]

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[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (c7c4cf7) 94.67% compared to head (6ef243e) 96.23%.

Files	Patch %	Lines
src/time_integration/methods_SSP.jl	92.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1677      +/-   ##
==========================================
+ Coverage   94.67%   96.23%   +1.56%     
==========================================
  Files         436      436              
  Lines       35166    35210      +44     
==========================================
+ Hits        33292    33882     +590     
+ Misses       1874     1328     -546     

Flag	Coverage Δ
unittests	96.23% <92.00%> (+1.56%)	⬆️

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[sloede]

LGTM, thanks!