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

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.