Fix MOI.is_valid for ConstraintIndex{VariableIndex,Parameter}

[odow]

Closes #414

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 94.39%. Comparing base (f5b6ecb) to head (cef2857).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #415 +/- ## ========================================== + Coverage 94.38% 94.39% +0.01% ========================================== Files 4 4 Lines 979 982 +3 ========================================== + Hits 924 927 +3 Misses 55 55 ```

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