randomSimpleGraph now supports generating trees

Carsten gave me the consent to make the changes. Along with this pull request, I have also sent the updated file to Carsten. I used the following code to check the updated function.

#include <ogdf/basic/Graph.h>
#include <ogdf/basic/graph_generators.h>
#include <ogdf/basic/simple_graph_alg.h>
using namespace ogdf;

int main()
{
	Graph G;
	int connected_count=0,tree_count=0;
	int n, m, max_edges;
	//randomSimpleGraph(G, 3, 4);
	for (int i=1;i<=10000;i++)
	{
		G.clear();
		n=randomNumber(1,200);
		max_edges=n*(n-1)/2;
		m=randomNumber(0,max_edges);
		randomSimpleGraph(G, n, m);
		if (isConnected(G)) connected_count++;
		if (isTree(G)) tree_count++;
	}
	std::cout<<connected_count<<" "<<tree_count<<endl;
}

It gives a non-zero tree_count.