fix: stoichiometric consistency due to SLIME rxns

[BenjaSanchez]

Description of the issue:

In the consistency tests from the memote report, the "Stoichiometric Consistency" dropped significantly after adding SLIME reactions, as these reactions are by definition mass unbalanced. We should find a way of fixing this, by e.g. modifying the chemical formulas of the backbone and/or chain pseudo-metabolites.

I hereby confirm that I have:

• [x] Tested my code with all requirements for running the model
• [x] Done this analysis in the master 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