Edit trait

Related to <a href="https://ebi-forecast.igb.illinois.edu/redmine/issues/2198">#2198</a> and <a href = "https://ebi-forecast.igb.illinois.edu/redmine/issues/2351">#2351</a>
This line is a little baffling.  After much investigation, I
figured out it is only needed so that @covariate is set when
rendering the new_covariate template inside the `add_covariate`
JavaScript function.  Of course, if @new_covariates is non-empty, the
@covariate variable will at least be non-nil (since it is set inside
the immediately preceding loop at line 124), but unless we get here
from the edit method (which sets @new_covariates to `[Covariates.new]`)
@covariate is going to be populated with non-blank data equal to the
last value it had in the loop, and we don't want this; we want to
start with empty fields for the new covariate we are adding.  Still,
it would be cleaner to set @covariate directly in the call to render
in the `add_covariate` function:

    jQuery('div#covariates').append('<%= escape_javascript(render :partial => "new_covariate", :object => Covariate.new, :as => :@covariate) %>');

Even better would be to use a local variable 'covariate' in the `new_covariate` partial in place of the instance variable '@covariate'.  Then we'd use

    jQuery('div#covariates').append('<%= escape_javascript(render :partial => "new_covariate", :object => Covariate.new, :as => :covariate) %>');

instead, and inside of the @new_covariates.each loop we could use

         <%= render(partial: 'new_covariate', object: c, as: :covariate) %>

and get rid of

    <% @covariate = c %>