Implement #elementAt for MElement

[rschnekenbu]

According to PR #394, it may be interesting to avoid duplication of method elementAt(int), created in this PR for MExecution, as it already exists for MLifeline.
Several other model classes may need it, so it would be interesting to shift this method to MElement.

[rschnekenbu]

Shifting to Milestone 3, as this is simply code improvement