Refactor: smoother conversion from SUNMatrixWrapper to SUNMatrix

[dweindl]

Adds an implicit conversion function to SUNMatrixWrapper make things more readable.

[codecov]

Codecov Report

Attention: Patch coverage is 63.15789% with 14 lines in your changes are missing coverage. Please review.

Project coverage is 77.78%. Comparing base (8b324bc) to head (91e6652).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #2317   +/-   ##
========================================
  Coverage    77.78%   77.78%           
========================================
  Files          318      318           
  Lines        20534    20535    +1     
  Branches      1436     1436           
========================================
+ Hits         15973    15974    +1     
  Misses        4558     4558           
  Partials         3        3           

Flag	Coverage Δ
cpp	73.57% <63.15%> (+<0.01%)	⬆️
cpp_python	34.01% <31.57%> (+<0.01%)	⬆️
petab	36.39% <26.31%> (+<0.01%)	⬆️
python	72.35% <63.15%> (+<0.01%)	⬆️
sbmlsuite	?

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

Files	Coverage Δ
include/amici/rdata.h	100.00% <100.00%> (ø)
include/amici/sundials_matrix_wrapper.h	76.54% <100.00%> (+0.29%)	⬆️
src/model.cpp	86.08% <100.00%> (ø)
src/newton_solver.cpp	70.86% <80.00%> (ø)
src/sundials_linsol_wrapper.cpp	15.31% <50.00%> (ø)
src/model_ode.cpp	66.05% <55.55%> (ø)
src/model_dae.cpp	31.16% <33.33%> (ø)

[FFroehlich]

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