allow to store process coordinates

this should be alternative to current process drawn in the center of the reaction line, import it from SBML:

Regarding the reaction node thing (item 3 below): In our format, we use a position for a reaction indicating where the reaction node should be drawn. The SBML layout extension doesn't provide such a thing as you observed. However, the ReactionGraph layout still can take a BoundingBox element together with a list of LineSegments, though the spec said that the BoundingBox should be ignored if LineSegments are provided. Do you think we can try to hack this by providing the Reaction node as a BoundingBox for the Minerva? It is easy for me to convert the position into a BoundingBox.

by wug[at]ohsu.edu