AGraph.copy() ignores subgraphs

[astrofitz]

The copy method does not preserve the subgraph structure (important for maintaining clusters).

This snippet reproduces the issue:

import pygraphviz as pgv
G = pgv.AGraph()
SG = G.add_subgraph(name='cluster_Test')
SG.add_node('A')
SG.add_node('B')
SG.add_edge('A','B')
print(G)

cG = G.copy()
print(cG)

producing output:

strict graph "" {
	subgraph cluster_Test {
		A -- B;
	}
}

strict graph "" {
	A -- B;
}