Network generation test model

Generates models using the new LSD internal network functions.

Use the code here as a blueprint for using the most common network macros.

Network structure in LSD is independent from the model structure (the 
parent-children relationship you create for the model's structure). Networks can
be created having any single object type (with the required number of copies) as
nodes (networks connecting different object types are not allowed). One model 
can have multiple networks simultaneously (even hierarchically, nets inside nets) 
but each object type can participate in one network at a time.

Links in the network connect the object copies that compose it. Links between 
nodes are always directed and can have optional weights. Undirected networks are
represented by pairs of links with opposite directions.

The NETWORK_LOAD and NETWORK_INI macros create the required network structure
(and even creates copies of the node objects as needed) by loading it from disk
(Pajek format) or by creating a pseudorandom network using one of the available 
algorithms.

Look at the 'case 8' example (star network) on how to manually create a network 
when an appropriate algorithm is not available. Check equation 'NewLink' to see
how to create new (random) undirected links in the network. Equation 'TopLink' 
is an example of more complex manipulation of existing links.

The NETWORKS_SAVE and NETWORK_SNAP macros allow for the saving of the existing 
networks to a file (Pajek formats), both as a single or as a series of snapshots.
