0.10.0b1

* 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