Give new database IDs to the kegg_additions

[eladnoor]

Now, we issue "fake" IDs that look like KEGG, with high numbers (e.g. C80067). This is confusing and should be changed to something like COCO:C00001 (COCO for COmponent-COntribution).

[eladnoor]

This options will be used only in cases where the compound has no InChI and cannot be found in any of the standard databases.
For example, "a quinone" is a class of compounds and therefore has no InChI, however we can use the MetaNetX identifier MNXM2499 instead of defining a new one.