Exchange reactions such as "DNA replication" should be removed from the list of potential exchange reactions

[hariszaf]

There are cases where reactions like "Protein biosynthesis", "DNA replication" and "RNA transcription" are added in the model.
This leads to their uptake in FBA for example; instead of using sugars from the media, the model just uptakes those.

We need to make transporters only for metabolites; manual curation on the list of possible exchange metabolites will probably be required.