Increase rigor of tests for model changes

[jonrkarr]

The tests for support for model changes basically just test that the simulation tool doesn't have a run-time when a SED model has SED changes.

The test suite should more rigorously verify that simulation tools implement model changes correctly.

• changeAttribute
• addXML
• removeXML
• changeXML
• computeChange
• setValue

Since the test suite can't directly observe model specifications, the test suite has to verify this by checking that simulation tools produce one or more data sets that demonstrate the impact of the model change. For example, a change could set an initial value of a model variable to zero, and then the test suite could verify that the first value of a data set for a data generator for the variable is zero, indicating that the change was correctly applied. This is challenging to do in a model language and algorithm agnostic way as this test suite aims to do.

[jonrkarr]

Done except for repeated tasks which is covered in #6