GitLab

The exponent applies to the multiplier as well.  Reorganize the `get_unit`
function to make it clearer what's the formula used.

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

Also:

* replace `@warn` with `@error`
* actually test the logging message in `readSBML`, instead of showing an extra
  message saying to ignore it.

how did it even pass before?

(Uses a few new models from EBI repository.)

this checks whether the fix of #16 works

well, who'd say it's hashing the filename?