add covariate on trait/edit

see <a href="https://ebi-forecast.igb.illinois.edu/redmine/issues/2198"> redmine #2198</a>
As I suggest above, you should only get here if _everything_ saves properly.  Otherwise you should re-render the "edit" action.

Note that even before your changes, there was a bug in the code whereby if the `@trait.update_attributes `call failed, then the code would try and fail to render the edit page.  The error message is "undefined method \`sites' for nil:NilClass".  The nil class in this case is the @citation instance variable, which is supplied by the edit method but isn't supplied in this method.  (I recently changed the template to need the @citation variable, but I neglected to set it in this method before calling `render :action => "edit"`.)  See commit number 423ee996 where I fix a similar problem in the yields controller.