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)
One other somewhat related thing I meant to mention is this: It seems a little weird to be instantiating new Covariate objects that never get saved to the database.  (This happens both here and also in the calls to render the `new_covariate` partial.  The only Covariate objects that ever actually get saved to the database are the ones instantiated in line 179.)  But since it seems such a convenient way to share the `new_covariate` partial between the code that displays form widgets for adding _new_ covariates and the code that displays the covariates that failed to get added when the `update` action throws an exception, it hardly seems worth objecting to.  For all I know, it's standard practice among Rails programmers to use model objects this way.