General cardinality feature updates

[mpsonntag]

This PR is a preparation for the introduction of Section-section and Section-property cardinality features to keep PRs at a size that is still convenient to review.

The PR introduces

• a generalization of the cardinality function for later reuse by different cardinalities
• catches some edge cases in the core cardinality function
• filters the validation warnings when checking for cardinality violations to display only warning messages specific to the validated object and not from connected objects as well.
• adds and updates tests where required

[coveralls]

Coverage increased (+0.02%) to 76.39% when pulling 6aa02e40c29da1ea0e2265eee50aeeed6730e1d5 on mpsonntag:cardUpdates into 7dde867ef857de9a66fbe2442f13d11a2edc2f69 on G-Node:master.