|  | dm | - the DMPlex object | 
|  | parentSection | - a section describing the tree: a point has a parent if it has 1 dof in the section; the section
offset indexes the parent and childID list; the reference count of parentSection is incremented | 
|  | parents | - a list of the point parents; copied, can be destroyed | 
|  | childIDs | - identifies the relationship of the child point to the parent point; if there is a reference tree, then
the child corresponds to the point in the reference tree with index childIDs; copied, can be destroyed |