Version 1.2.0

• search for GAMS in more places, especially on Windows
• gdxpds.gdx.GdxSymbol.unload method for user-directed memory managment
• convenience functions gdxpds.gdx.append_set and append_parameter
• special value handling moved to its own module
• clarifying that Python 2.X is not longer supported
• general documentation, pylint, and testing improvements