fix bug, issue #2093

[hariszaf]

Please include a short description of enhancement here

This PR fixes the bug described in issue #2093. The minimum flux of the objective function of species 1 is now calculated as supposed to.

I hereby confirm that I have:

• [x] Tested my code on my own machine
• [ ] Followed the guidelines in the Contributing Guide
• [x] Selected develop as a target branch (top left drop-down menu)