CR — Create classes to hold entry and exit criterion references

Currently, entry and exit criteria are just a list of sentries references. Since these criteria are depicted in a CMMN diagram, it would be easier to refer them if they had their own ids. To do so, they should become first class citizen that inherited CMMNElement. This would also allow to use an exitcriterionRef in PlanItemOnPart instead of a Sentry ref (which was the original intention anyway.)

Create classes to hold entry and exit criterion references

1. Modify figure 5.6 to include Criterion, EntryCriterion and ExitCriterion classes
2. Modify table 5.18
3. Add section 5.4.5.1 Criterion
4. Add section 5.4.5.2 Entry Criterion
5. Add section 5.4.5.3 Exit Criterion
6. Modify figure 5.8
7. Modify table 5.26
8. Modify figure 5.7
9. Modifiy table 5.22
10. Modify XSD to add new elements entryCriterion and exitCriterion
11. Modify tPlanItem complexType in the XSD
12. Modify tStage complexType in the XSD
13. Modify tPlanItemOnPart in the XSD