Update constraint indexes when adding constraints without parameters

[gvidigal-psr]

When alternating between constraints with and without parameters, the model.constraint_outer_to_inner Dict would lose track of the constraint indexes.
In the new test, without the changes, the dual for c2 returns -3.0.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.18%. Comparing base (275ef07) to head (cff4eca).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #156      +/-   ##
==========================================
+ Coverage   95.14%   95.18%   +0.03%     
==========================================
  Files           5        5              
  Lines        1010     1018       +8     
==========================================
+ Hits          961      969       +8     
  Misses         49       49              

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