Hi, A corresponding class to support `GAMS.GAMSVariables` is missing in [gams.py](https://github.com/erkkar/GDX2py/blob/master/gdx2py/gams.py). This causes an [issue](https://github.com/Spine-project/Spine-Toolbox/issues/1741) of SpineToolbox, where its importer cannot handle `Variable` type data from .gdx properly.
