Add VariantMembership capacities on variation elements

[jessymallet]

#Context

SysML include a fundamental aspect which is called variability. It includes the definition of variation and variant (specification https://www.omg.org/spec/SysML/2.0/Beta2/Language/PDF p.64).
A variation (which can be applied to any kind of definition or usage) identifies an element in a model that can vary from one design configuration to another. Variants are usage elements containing by the variation.

Objective

If a Definition has isVariation = true, then it represents a variation point Definition. In this case, all of its members must be variant Usages, related to the Definition by VariantMembership Relationships.

When a user set the property "Is Variation" to true (by using details view or direct edit), all of its member should be moved in a new VariantMembership.