improve sbml3 performance
improve sbml3 performance

* lxml is actually slower than cElementTree and removed
* Give sympy a list of locals - makes faster and also more stringent (an
  error will now result when trying to make a reference to an
  nonexistent gene).
* Also comply with draft spec change and make reversibility correct

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