allow other RHS in analysis callback

[ranocha]

This fixes the problem brought up in #1202 (comment). It allows us to use an ODE RHS different from our rhs! function. This can be very nice, e.g., for hyperbolic-parabolic systems set up as SplitODEProblem with rhs! and rhs_parabolic! in #1149 or when the user wants to modify the RHS call as in https://discourse.julialang.org/t/ann-trixi-jl-v0-3-sciml-integration-and-a-new-modular-approach-for-easy-extension/50419/43.

I made this a PR to main instead of dev or so since it is generally useful and should be shipped right now.

This is a cleaned-up version of #1204.

[sloede]

LGTM!

[codecov]

Codecov Report

Merging #1205 (02a1c22) into main (22348a7) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1205   +/-   ##
=======================================
  Coverage   96.78%   96.78%           
=======================================
  Files         310      310           
  Lines       24284    24284           
=======================================
  Hits        23501    23501           
  Misses        783      783           

Flag	Coverage Δ
unittests	96.78% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/time_integration/methods_3Sstar.jl	95.88% <ø> (ø)
src/callbacks_step/analysis.jl	96.27% <100.00%> (ø)
src/time_integration/methods_2N.jl	93.51% <100.00%> (ø)

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