Suggestion: function to convert from 3.0 to 2.0 to be provided #50
Description
Given that SbgnUtil.readFromFile can read from both 2.0 and 3.0, but SbgnUtil.writeToFile exports to 3.0 only, I think it would be helpful that libSBGN provides conversion to 2.0 as well for the output file. In this way, the output file could be used directly by softwares/tools that currently support SBGN 2.0 only.