Refactor relations

[JaZo]

Description

I refactored the relations by adding an interface for singular and plural relations.

Motivation and context

All relations behave slightly different and this PR aligns their behaviour. This should improve the code quality.

How has this been tested?

Tested using existing unit tests.

Types of changes

What types of changes does your code introduce? Put an x in all the boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

Go over all the following points, and put an x in all the boxes that apply.

• I have read the CONTRIBUTING document.
• My pull request addresses exactly one patch/feature.
• I have created a branch for this patch/feature.
• Each individual commit in the pull request is meaningful.
• I have added tests to cover my changes.
• If my change requires a change to the documentation, I have updated it accordingly.

[bbrala]

Code seems fine and more consistent this way. Well done.

[JaZo]

Code seems fine and more consistent this way. Well done.

N.B. Based on the feedback from Scrutinizer I've removed RelationInterface in favor of OneRelationInterface and ManyRelationInterface as they are actually different and RelationInterface should not be used standalone.