improve ability to handle symbolic stoichiometry
improve ability to handle symbolic stoichiometry

If a symbolic stoichiometry cannot be evaluated, it is assumed to be
positive.

zakandrewking/cobrapy,JuBra/cobrapy,aebrahim/cobrapy,JuBra/cobrapy,zakandrewking/cobrapy,jeicher/cobrapy,aebrahim/cobrapy,jeicher/cobrapy