Activate model env before calling RelationshipClass and Parameter

[manuelma]

Checklist before merging

• Documentation is up-to-date
• Unit tests have been added/updated accordingly
• Code has been formatted according to SpineOpt's style
• Unit tests pass

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.02%. Comparing base (6b8647b) to head (95fa1fb).
Report is 2 commits behind head on v0.7.2.x-release.

Additional details and impacted files

@@                 Coverage Diff                  @@
##           v0.7.2.x-release    #1096      +/-   ##
====================================================
+ Coverage             85.97%   86.02%   +0.05%     
====================================================
  Files                   141      141              
  Lines                  3750     3765      +15     
====================================================
+ Hits                   3224     3239      +15     
  Misses                  526      526              

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