rewrite Formula
rewrite Formula

* More robust, works under py2/3
* Faster because weights are calculated lazily (they are a @property)

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