trait edit

Related to <a href="https://ebi-forecast.igb.illinois.edu/redmine/issues/2351">#2351</a>, <a href="https://ebi-forecast.igb.illinois.edu/redmine/issues/2198">#2198</a> and <a href="https://ebi-forecast.igb.illinois.edu/redmine/issues/2245">#2245</a>(only the second part; the first part has already been merged in a previous pull request)
I meant to mention this earlier: Since the TraitsController object is always re-instantiated with every HTTP request, `@new_covariates` is always nil upon entering the `edit` action method.  So you might as well just write
```
@new_covariates = [Covariate.new]
```
This is a little confusing, but I think I'm right about this: Although `@new_covariates` is set in the `update` method, and although it passes along that value to the `edit` template if that method throws an exception, the code in the `edit` method (_this_ method) doesn't get run during this process.