Do not replace all constraints by default values automatically if one constraint only is missing

[Bachibouzouk]

Fix 161

Changes proposed in this pull request:

• The constraints are not all set to default values if only one constraint is missing, only the missing constraint is set to default value
• The default values for the constraints are now located in src/constants.py under the variable DEFAULT_CONSTRAINT_VALUES

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• [x] Update the CHANGELOG.md
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• [x] Check if benchmark tests pass locally (EXECUTE_TESTS_ON=master pytest)

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