Member predicate MetricElement::getALevel

An element has no level defined if it is cyclically dependent on itself. Otherwise, it has:

• level 0, if it does not depend on any other elements,
• level 1, if it does not directly depend on another element that occurs in the source, and
• level n+1, if it depends on another element at level n.

Note that according to this definition, an element may have multiple levels.