Resolve "Check static contract always return Ok even in case of invalid model"

This MR does some clarification in the static checkers and how to start them.

We now have 3 distinct static checkers (with specific error markers)

• EMF Structure (check about multiplicities defined in the ecore file)
• Requirement Contract checker : check all the static contract associated to at least one requirement
• Domain Contract checker : check all the static contract that are NOT associated to a requiement, they are supposed to be domain specific

each of these checker can be called individually via dedicated popup (in the advanced sub menu) a full check is available as a popup menu

This PR also improves the mini example to illustrate that.

Closes #405 (closed) Closes #393 (closed)