Use .png for objecttree in docs (closes gh-798)
Use .png for objecttree in docs (closes gh-798)

FrancescAlted/PyTables,PyTables/PyTables,PyTables/PyTables,PyTables/PyTables,avalentino/PyTables,avalentino/PyTables,avalentino/PyTables,FrancescAlted/PyTables,FrancescAlted/PyTables