[Graph Generation Algorithms] Erdős–Rényi Graph

[MatheusFarias03]

This PR is related to #277 and adds a new function age_create_erdos_renyi_graph that generates a random graph with n nodes, where the probability for each node to connect to another one is based on a given probability. The edges can be set to be in one direction or in both directions. It is also possible to set the labels for both vertices and edges.

It uses the G(n,p) model, where the graph is constructed by connecting labeled nodes randomly. Each edge is included in the graph with probability p, independently from every other edge. Equivalently, the probability for generating each graph that has n nodes and M edges is

Syntax:

ag_catalog.age_create_erdos_renyi_graph(graph_name name, 
                                                n int, 
                                                p name,
                                                vertex_label_name name = NULL,
                                                edge_label_name name = NULL,
                                                bidirectional bool = true)

Additionally, the probability p was set to be of type name because neither PG_GETARG_FLOAT4(2) or DatumGetFloat4(PG_GETARG_DATUM(2)); were working.