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closes #119

Closes #133

fixes #124

* `getS` -> `stoichiometry_matrix`
* `getLBs`, `getUBs` -> `flux_bounds`
* `getOCs` -> `flux_objective`

The function
[`SBMLDocument_convert`](http://sbml.org/Software/libSBML/5.18.0/docs/formatted/c-api/class_s_b_m_l_document__t.html#a96677d1b2225c23f3b3ab662a4e547e2)
returns 0 when it is successful, other numbers otherwise.

This should hopefully be slightly more future-proof, as long as libsbml doesn't
change the strings.

fixes terminal flood on many existing models

closes #117

Oh my, like, this is the 5th implementation of ifelse we have here?

Fixes #112

This rationalise the way we report errors from libsbml in a few different
places.

Fixes #100

- standalone literals return Num type
- constants are now converted properly

fixes #102

@exaexa is there a place we can view all the names and add more? we are probably missing a bunch