Convert all test models to SBML

[gregmedlock]

Looks like this will require updating all of the test models to account for COBRApy updates. To prevent future changes from breaking test models, all test models should be encoded as SBML (rather than pickled ensembles or COBRApy json objects). I will make these updates to the test models.

Originally posted by @gregmedlock in #59

To avoid compatibility issues as COBRApy gets updated, test models specific to medusa should be saved as SBML files in the test directory. Where possible, use the standard COBRApy test models instead of including new ones in the medusa test directory.