Make lastReturnCode mutable

[hedtke]

Many methods were already marked as const. The member m_lastReturnCode is always indirectly modified via m_scipCallWrapper.

Now the member m_lastReturnCode is marked mutable and all methods that can be const are now const.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.05%. Comparing base (6ab0c12) to head (33b2e42).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #40   +/-   ##
=======================================
  Coverage   98.04%   98.05%           
=======================================
  Files          10       10           
  Lines         205      206    +1     
  Branches       57       58    +1     
=======================================
+ Hits          201      202    +1     
  Misses          3        3           
  Partials        1        1           

Files with missing lines	Coverage Δ
include/scippp/model.hpp	95.83% <100.00%> (ø)
source/model.cpp	98.80% <100.00%> (+0.01%)	⬆️

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[wienkamp]

Only one comment not related to the changes in the MR: I cannot reliably build the 'tests' target with the current project setup: If I first follow the instructions to build the tests target (conan v2, CMake > 3.19) everything works. However, if I first follow the instructions for building the ScipPP target and subsequently want to build the tests target, the error "No rule to make target tests" is thrown. This can be fixed by using tc.cache_variables["BUILD_TESTS"] instead of tc.variables["BUILD_TESTS"] in the conanfile.py. Same holds true for other variables set in the conanfile.py.