* Fixes
- improvements to displaying objective functions
- better notebook intergration for plotting optimization results
- Debug loading medium from a DataFrame
- add is_exchange property to reaction
- exchange reactions can only have one metabolite (not zero)
- fix remove reactions
- Improvements to DifferentialFVA
- fold change calculation and representation in notebook
- differential FVA implementation to yield feasible designs.
- improvement: methods that change the objective inside should be
tested to ensure that the objective remains the same
- Speedup FVA by doing minimization first then maximization
* Features
- support multiple substrates on yield functions