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 (94e90ba).

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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