Incorporate tests of support for model languages, numerical accuracy, and reproducibility

[jonrkarr]

The current tests focus on support for SED-ML, COMBINE, and the BioSImulators conventions. The current tests implicitly assume that the simulation tools have already been tested for correctness, ideally using something like the SBML test suite. However, this is not guaranteed.

Part of the reason why the test suite doesn't currently test numerical results more deeply is that this requires tests for each model language, and BioSimulators is intentionally architected to support any language. As a result, its not trivial to try to build rigorous tests for each model language.

Nevertheless, the test suite could be more powerful if merged with something like the SBML test suite.

• Reproducibility (multiple executions produce the same results)