This commit introduce two small graph structures to treat specific weighted
nodeTo graph are made of nodes that know there outgoing edges
nodeFrom graph are made of nodes that know there incoming edges
Both class are constituting a graph by using eXlibris DATAMANGER concept
to associate to key a graph node.
Both class hold some information on nodes and edges.
Typical usage of those graph are linked to mesh traversal. For example
if the keys (graph nodes) are vertex we can create a graph that impose
mesh traversal through edges of this graph that can be mesh edges or
mesh element or ...
Or if the keys (graph nodes) are elements we can create a graph that
impose traversal through edges of this graph that can be element
connection, .... And this way the traversal in the mesh is done
from mesh element to mesh element.
This library is very experimental and its API is subject to changes in