SBGN-ML import: modified simple chemical (metabolite) glyphs

The new specification will have it as a stadium shape for simple chemicals, as on the first image. Right now, according to the current SBGN PD spec, it is an ellipse (as in CellDesigner). Still, not just a circle.

Original view in yEd:

After being converted from GraphML to SBGN-ML and opened in SBGN-ED:

Resulting view in MINERVA:

Please ignore the arrows, I'll describe that as a separate issue.