replace sympy with ast in sbml3 writer
replace sympy with ast in sbml3 writer

One less third party dependency. Also much faster.

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