Bug: Validator: `mappings.a->sum(element.value)` (variable access) should be forbidden within the (global) objective

[maxkratz]

Loosely related to #220.

See the newly created test case: https://github.com/Echtzeitsysteme/gips-tests/blob/eb501311d486dd079da6357ef11f8fcbbf9ade55/gipsl.all.build.objective.keywordaccess/src/gipsl/all/build/objective/keywordaccess/Model.gipsl

Either the validator should completely forbid the access of (M)ILP variables within the global objective, or the generated code should compile fine. I weakly vote for the first solution.