Eliminate or reconsider the concept of clone markers in SBGN

[draeger]

The concept of clone markers is problematic and should be abandoned or changed entirely for the following reasons:

1. These markers always highlight exactly the wrong parts of the network. In a large-scale, realistic network, we expect many currency metabolites, such as ATP or NADPH, and so forth. So, it is hard to keep track of the primary metabolites. Clone markers, however, highlight the distracting secondary metabolites.
2. Their shape is inappropriate because they fill part of the node, making it difficult to overlay SBGN maps with measurement data. If you want to display any quantity, e.g., the concentration of some metabolite, you should remove the clone marker if it conflicts with the visualization.
3. They are not universally applicable: The current specification does not allow clone markers for some glyphs.

I suggest rethinking this concept and considering removing it entirely or highlighting those glyphs that are not cloned rather than those that are cloned.

Originally posted by @adrienrougny in #127 (comment)