duplicate metabolites identified

Description of the issue:

Reported by Han, Yichao [email protected], Thanks for identifying this! There seems two metabolites suspected as duplicates: s_3948 and s_0557

I hereby confirm that I have:

• [x] Tested my code with all requirements for running the model
• [x] Done this analysis in the develop branch of the repository
• [x] Checked that a similar issue does not exist already
• [x] If needed, asked first in the Gitter chat room about the issue

Basically s_0557(D-fructose 6-phosphate) contains s_3948(beta-D-Fructose 6-phosphate) and another alpha isomer. So technically, they are not 100% same. In the future, I think we will add all isomers in the model, so for now, it is ok to keep both now as far as I know. But it is good that to document that we have noticed this issue.