fix mat serialization for reversibility
fix mat serialization for reversibility

Fixes #158

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