0.8.0b1

* Features
- Support multiple reactions in ratio constraints
- Yield based phenotypic phase plane (aka production envelope)
- Support for knocking metabolites
- Add with statement in ProblemCache
- Implementation of OptSwap

* Fixes
- Faster retrieval of shadow prices from solver
- Debug load model
- Faster lmoma and moma
- Debug gene knockout
- Fix broken unit-tests that depended on comparing dataframes