New unary operator in AQL: level by namespace

[amir-zeldes]

Currently #1:root only finds nodes that are the root of a graph in all components. Often we want to know what the root of a subgraph is, e.g. just the dependency annotation, even though the same node is not a root if all context components are considered. Since we have information in the rank table on the depth level of each node in each component, it is proposed to allow level queries for any level (not just root), and limited to any specific namespace. The root operator should not be abolished despite this. Some examples:

node & korpling/ANNIS#1:level=1  (any node at level 1 in any context component)
node & korpling/ANNIS#1:level=1,3 (any node at level 1,2 or 3 in any context component)
node & korpling/ANNIS#1:dep:level=0 (any node which is the root of a context component in the namespace dep)