add covariate on trait/edit

see <a href="https://ebi-forecast.igb.illinois.edu/redmine/issues/2198"> redmine #2198</a>
   Although using a redirect here avoids the problem of the instance
    variables needed for the edit template being undefined (since the
    "edit" method guarantees that they are), it has some serious
    drawbacks compared with using 'render :action => "edit"'.

First, all of the values that the user entered into the form are
    thrown away (even the correct ones), so the user has start all
    over again in filling out the form.

Second, the @trait.errors property is emptied out so that the line
    in the template (<%= f.error_messages %>) that would normally
    display the trait validation results doesn't have anything to
    display.  So the user both has to start over from scratch and
    doesn't have some needed information about what might have gone
    wrong.

Getting "render" to work is really rather simple here: we just
    have to set the @citation instance variable, and we can get the
    value from the @trait instance variable:

          @citation = @trait.citation