fix bug in check_mass_balance
fix bug in check_mass_balance

elements have moved from Formula.elements to Metabolite.elements.
However, when loading old pickles, they might still be in
Formula.elements, so that will still be checked in the case of an
AttributeError.

This fixes #162

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