Creates an acyclic network with the given size and average degree. Node
degrees follow a Possonian distribution.
     
    
    Usage
    acyclic_network(size, avg_degree, reduced_root_degree)
 
    
    Arguments
- size
- number of nodes 
- avg_degree
- average number of neighbour each node has 
- reduced_root_degree
- if true, the degree of the root node is reduced by one