Add default parameter handling for macro elasticity simulation

[lucassz]

Add default parameter handling for macro elasticity simulation. Follow-up to #883 and #905: currently, running the macro elasticity model without inputting any data results in not even a UI error, but an uncaught server error in converting an empty string to a float. This adds default handling in a general way. However, it does make the assumption that there is only one column/value per input; this was already implied in the previous code, and I believe that should be okay for this model, but please let me know if it is not.

[hdoupe]

@lucassz Nice catch. Would you mind adding a test in the dynamicapp where a reform is submitted with the defaults?

[lucassz]

@hdoupe This is done. I had to make the class not be a TestCase to get the parametrization to work.

[hdoupe]

@lucassz what do you think about dropping the assert* methods? When I removed the TestCase inheritance from some of the other test classes, I dropped these methods because I think it's easier to just read a==b instead of self.assertEqual(a, b).

[lucassz]

Yes, I think that is a better idea. I was trying to minimize changes to the code, but will make the change.

[hdoupe]

LGTM. @lucassz are you ready for this to be merged?

[lucassz]

@hdoupe Seems good to me too.

[hdoupe]

Cool, thanks for the contribution.