store ec-model in yml format

[edkerk]

Currently, lossless export of ec-models can only be as .mat files, as SBML does not have fields to store many of the additional fields. The new model.ec structure would be even more problematic. However, it would be good to have a flat-text lossless file format for export and import of ec-models.

We can use YAML (expanding on RAVEN's writeYAMLmodel), as we can easily define the additional fields, the I/O of the YML file is faster than libSBML, and the file format is very suitable for diff-ing.

Most of model.ec will be easy to implement, the only tricky part might be rxnEnzMat. Probably most sensible is to use a structure that is similar to how reaction stoichiometry is included in YML:

    - !!omap
      - id: "r_0310"
      - name: "cystathionine g-lyase"
      - metabolites: !!omap
          - s_0178: 1
          - s_0419: 1
          - s_0803: -1
          - s_0980: -1
          - s_0981: 1