* store the entity tag on which the element is classified in the element cache: this allows to retrieve the entity when accessing the elements by tag (the entity pointer is not stored in MElement to limit memory use)
* enhance mesh/getNode() and mesh/getElement() API to return the entity on which the node/element is classified

With these additions one can e.g. identify entities (say, a meshed volume) by coordinate, by calling mesh/getElementByCoordinates() followed by mesh/getElement()