graphml to png tool

[Leahh02]

Right now there's no tool to convert a graphml to a png.

The generate_viz function from beeflow/common/gdb/generate_graph.py can be called to do this, but maybe there should be a tool to make this easier. A tool like this can be helpful when the neo4j database misses some information that causes the creation of the graph in beeflow/common/gdb/generate_graph.py to fail.